Concepts of epidemiology : integrating the ideas, theories, principles, and methods of epidemiology
Concepts
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Alexander Bruce and John Usher Professor of Public Health, University of Edinburgh, and Honorary Consultant in Public Health, NHS Lothian Board, UK formerly
Professor and Head of the Department of Epidemiology and Public Health, University of Newcastle Upon Tyne, UK
1
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Dedication to the Third Edition
I dedicate this third edition to my wife Roma, my sons Sunil, Vijay, Anand, and Rajan, my daughter-in-law Hannah, and my grandson Leo, for their love, which enriches my life.
Dedication to the Second Edition
I dedicate this second edition to my brothers Sohan, Kalwant (deceased 1989), Jaswant, Surinder and Mhinder, and my sisters Harbans, Sheila, and Baljinder for their love, support, example, and direction.
Dedication to the First Edition
I dedicate this book to my mother Bhagwanti Kaur Bhopal† for impressing on me (and my siblings) the importance of education, and encouraging us to make up for her own lack of schooling and formal education; and to my father Jhanda Singh Bhopal† for setting an example of how to work hard, shoulder responsibility, and strive for self-improvement.
†(deceased 2010)
Foreword
Foreword to the Third edition
A concept is a general idea or understanding, usually of something abstract rather than concrete. Raj Bhopal’s Concepts of Epidemiology conveys the general ideas and understanding of epidemiology supremely well, perhaps better than any other monograph on the discipline of epidemiology. It has the added virtues of being clearly and concisely written, and provides comprehensive cover of all essential aspects. I thought the first edition was the best book of its kind that I had ever read. The second edition was even better; and now this third edition clarifies the ideas and enhances understanding even better than ever. It is a book that ought to be bedside reading for all epidemiologists everywhere.
John M. Last Emeritus Professor of Epidemiology University of Ottawa
Foreword to the second edition
The exciting, innovative features of Raj Bhopal’s book are the emphasis throughout on a conceptual approach and the systematic focus on underlying concepts and fundamental principles. This approach leads students of epidemiology, the intended readers of the book, to a logical understanding of the methods and procedures used in epidemiological research and practice. That was why I endorsed the first edition with unqualified enthusiasm.
The second edition is a thorough revision of the first. There is extensive rewriting and updating throughout the text, which remains crystal clear, easy to read, and easy to understand. The details are specified in the Preface. Students of epidemiology who use this book to enter upon their study of epidemiology have every reason to be grateful to Raj Bhopal for making their task easier and more pleasurable than it might otherwise be.
I am happy to endorse this new edition with the same unqualified enthusiasm with which I greeted the first edition.
John M. Last Emeritus Professor of Epidemiology
University of Ottawa
Foreword to the first edition
When I was learning epidemiology in the late 1950s, I was inspired by the first edition of Jerry Morris’s now—classic little monograph, Uses of Epidemiology, but there were hardly any comprehensive current textbooks to guide me. The few available books were either unreadable, unhelpful, or failed to orient my thoughts along the directions my ideas were taking. How times have changed! Now there are so many good books that it is difficult for the uninitiated to select the most suitable one to meet their needs. In 1997, Raj Bhopal reviewed 25 textbooks of epidemiology, discussing their approach to the subject, and their strengths and weaknesses, in a critical
commentary that is helpful to teachers and learners alike. Now, from the University of Edinburgh (where I spent five happy years in the 1960s), Raj Bhopal, who holds the Usher chair of public health, has written his own introductory textbook for graduate students who are embarking upon the detailed study of epidemiology.
This is an excellent introduction. Raj Bhopal’s approach is conceptual—he describes and explains the underlying concepts and methods of epidemiology with clarity and with apt examples, and simple, elegant illustrations. Frequently throughout the text he asks penetrating questions that will test the limits of his readers’ intellectual capacity—an admirable feature that other authors could copy with benefit to themselves and their readers. All the essentials are here: the person–population dyad, variation, error, bias, confounding, causality, the spectrum of disease, the ‘iceberg’ concept, risk and its relationship to disease frequency, study design, the ethical framework within which we practise epidemiology and conduct research, the relationship of epidemiology to other scholarly pursuits, and finally, some thoughts about the way the discipline has evolved and is likely to continue to evolve in the lifetime of those now entering upon careers in this field.
If I may speak directly to students starting the study of epidemiology: you can be grateful that there is a book like this to guide you along the fascinating pathway that leads to epidemiological enlightenment and understanding. This book will enable you to comprehend the connections between individual and population health, the natural history of disease, the methods of epidemiology, the interventions that work and don’t work, and the role of epidemiology as the fundamental public health science. This is a book for you to buy, to read, to study, and to enjoy.
John M. Last Emeritus Professor of Epidemiology University of Ottawa
Preface
Preface (incorporating that for the first and second editions)
After the first edition in 2002 and given the durability of concepts and methods I thought I could rest up, perhaps for ever! Instead, numerous changes and additions were required for a second edition only six years later. I was amazed to find so much new to do for the third edition only seven years on.
A concept is a complex idea, or a cluster of interrelated ideas. A concept lies behind the word or phrase we use to describe something. This book aims to explain and illustrate the ideas underlying the language, principles, techniques, and core methods in epidemiology.
I explain the key concepts underpinning the science of epidemiology and its applications to research, policy-making, health-service planning, and health promotion. I emphasize theory, ideas, and epidemiological axioms to counter the criticism that epidemiology is an atheoretical discipline.
There are 10 chapters. Many introductory courses are designed around 10–15 or so sessions. I envisage that the core of this book could be grasped in 10 days of committed study, preferably in the context of a taught course, but also independently. I have written the book with the special needs in mind of students who are studying independently or online. Appendix 1 provides some ideas for teachers on potential curriculae for taught courses for undergraduate and postgraduate students and in continuing education.
The learning objectives introducing each chapter are expressed mostly in terms of the reader acquiring understanding. I believe that understanding is the highest form of learning, from which may flow durable knowledge, changes of attitude, and the achievement of skills. Understanding often comes from solving problems and answering questions (i.e. active learning). I have incorporated numerous challenging exercises, some of them requiring basic mathematical concepts and calculations. Mostly, these are embedded in the text but some conclude each chapter.
This book is deliberately thorough in its discussions, and not simply brief and descriptive, and this is to help readers to achieve a deeper understanding and to help bridge the world of theory to that of practice.
The book places emphasis on integrating the ideas of epidemiology. The interdependence of epidemiological studies and their essential unity is a particularly important theme.
In the first edition I stated that this book differs in many ways from alternatives, for example:
◆ The concepts of epidemiology are discussed in detail, and in an integrated way.
◆ The concepts are dominant whereas in other books the methods dominate.
◆ The epidemiological idea of population is explicitly the foundation of the whole book. In most other books, the population idea is implicit, and in some it is neglected.
◆ The practical applications of each concept are considered, and illustrated with examples drawn from contemporary research and public health practice, including health-care policy and planning. The idea is that the reader will acquire the depth of knowledge to use the concepts and not merely be aware of them.
◆ The work is rooted in the basic ideas of the science of epidemiology, which are wholly applicable worldwide.
◆ The emphasis is on gaining understanding, and not on calculations, except where this is essential to understanding.
◆ Most of the exercises require reflection not calculation.
I think this analysis is still correct.
Second Edition
In preparation for the second edition, I looked to criticisms and comments of book reviewers, previous students, and co-teachers. Surprisingly, the book earned much praise and little criticism. Nonetheless, I made many changes, some of them in response to specific feedback, including:
◆ simpler writing, particularly to meet the needs of students whose main language is not English, and those without a medical/clinical background;
◆ several new questions, answers, problems, and exercises, including 20 questions in Chapter 9 to develop students’ understanding of study design;
◆ a brief introduction in Chapter 1 to incidence, prevalence, and study design;
◆ a set of questions with model answers so students can test themselves at the end of each chapter;
◆ a brief introduction to genetic epidemiology;
◆ extra concepts including correlation/regression, regression to the mean, and regression dilution bias, effect modification and interaction, receiver operating curves, the counterfactual approach, and birth cohort analysis in relation to cross-sectional and secular trends;
◆ an introduction to the purposes and principles of reviews, systematic reviews, and meta-analysis;
◆ a clearer exposition of epidemiology’s relation to statistics, based on the principles underpinning epidemiological analysis of data;
◆ expansion in many directions including on epidemiological variables and data privacy.
Third Edition
My focus remains on the ideas, strategies, methods, and applications of the discipline. What added value might readers expect in this revision? In addition to more rigorously edited and updated and yet expanded text, with further simplification of language, readers will find:
◆ The use of exemplars, highlighted in panels, to illustrate the concepts and methods under consideration in every chapter
◆ Much more emphasis on causality, and an introduction to the language and approaches of causal graphs
◆ Introduction to the language (including an expanded glossary) and approaches of the directed acyclic graph (DAG), translating the technical terms into more familiar epidemiological words and phrases (with an exemplar)
◆ More and deeper consideration of the qualities of epidemiological variables
◆ Expansion of previously introductory material on how new genetic technologies are changing epidemiology, with special emphasis on the Mendelian randomization study (with an exemplar)
◆ Fuller consideration of the technical, ethical, conceptual, and practical epidemiological issues arising from computerized linkage of records (with exemplars)
◆ A clearer explanation of the way one moves from data to information and intelligence through systematic analysis and interpretation
◆ A clearer exposition of difficult ideas including non-differential misclassification, effect modification, and its statistical equivalent of interaction
◆ An expansion of the study design chapter
◆ In line with trends in epidemiology, a clearer distinction between proportions and rates, including adoption of distinctive terminology when appropriate
◆ Consideration of the potential implications for extremely large influence on tens of millions, if not hundreds of millions, of people
The old favourites in previous editions are still there; for example, thought exercises embedded in the text, numerical calculations, historical references (also see Appendix 2 for further reading and useful websites), and the links between epidemiology and both clinical and public health practice. Readers acquainted with the first edition will recognize the style, and hopefully they will agree it has matured and improved, and not just grown fatter over time.
I meet readers of Concepts of Epidemiology in all corners of the globe and I am heartened by their remarks. They tell me that this book explains complex issues simply. I have worked to ensure this third edition will make difficult ideas even easier. Let me know whether you agree (email me at raj.bhopal@ed.ac.uk).
Happy reading!
Raj Bhopal December 2015
Acknowledgements
Acknowledgements for the Third Edition
I will never forget my debts to those who have helped me to write this book, and they are recorded in the Acknowledgements for the first and second editions (abridged). This page, as a result, is short. First, I thank my secretary Anne Houghton whose help in a myriad of tasks made this revision possible. The motive to take on the task came from OUP’s commissioning editor and much help was given by Caroline Smith and James Cox.
Dr Debbie Lawlor has given me many insights into the topic of interaction. Dr Snorri Rafnsson, Dr Markus Steiner and Dr Colin Fischbacher’s (co-teachers) have helped in my new writings, the latter two especially on causality and DAGs. Professor Harry Campbell checked the new writings on Mendelian randomization. I am solely responsible for any errors or misinterpretations.
I thank my students, colleagues, and fellow writers across the world. I am indebted to the University of Edinburgh for providing the challenging yet stable academic environment required for this kind of endeavour. Permissions for reproducing others’ work are recorded separately, but I also express my gratitude here. I thank John Last for his generous foreword.
Finally, I have dedicated this edition to my immediate family, which is the least they deserve.
Acknowledgements for the Second Edition (abridged)
OUP has proven itself a most supportive publisher with many highly professional colleagues— I thank especially Helen Liepman, Georgia Pinteau, Rosemary Bailey, and Claire Caruana for their feedback, encouragement, and help over the years that has spurred me to prepare this edition. Professor John Last’s support has been constant and reflected in his splendid second Foreword for me. Dr Colin Fischbacher’s comments on all the new sections were invaluable for he has a keen and critical eye. My postdoctoral colleague Dr Charles Agyemang gave prompt, enthusiastic, and able feedback on the new material. Professor Harry Campbell offered expert guidance on my introduction to genetics. Professor Richard Heller provided excellent material on population impact numbers, so if my distillation has impurities, this is my responsibility. Dr Sarah Wild helped me think about the section on interaction.
My family is now used to me and my scribbles. I thank my children Sunil, Vijay, Anand, and Rajan for the joy and fulfilment they give me. Without the tolerance and love of my wife, Roma, I would not have had the peace of mind, focus, and drive to pursue my academic passions. In trying to thank her I am, for once, lost for words!
February 2008
Acknowledgements for the First Edition (abridged)
My foremost debt is to the innumerable people who have taught me, whether in the classroom, seminar, and conference, or by their writings. One absorbs ideas and facts from others and over time synthesizes them with one’s own thoughts and experiences. Eventually, it is impossible to distinguish one’s own ideas from those of others. If my readers recognize their own ideas, and think they are incompletely acknowledged, then please accept my thanks and let me know.
Marcus Steiner, an MSc (Epidemiology) student and graduate at Edinburgh University, was my postgraduate reader-critic and helper. His advice and help, particularly with the technical preparation and development of the figures, was immensely useful.
Dr Sonja Hunt stimulated me to think about Figure 10.1. Dr Colin Fischbacher helped me proofread the manuscript and made several thoughtful observations. Professor John Last enthusiastically agreed to write the Foreword.
Others who provided academic advice on one or more specific chapters include: Professor Carl Shy, Dr David Chappel, Dr Eileen Kaner, and four anonymous referees who commented on the book proposal and chapter outlines.
I thank Dr Mike Lavender for permission to use extracts from our joint unpublished paper on the role of epidemiology in priority setting, and co-authors on various publications that I have drawn upon (these are referenced). I have drawn heavily on my publications to prepare Chapter 10 and Acknowledgements, details of which are given below.
Helen Liepman, Commissioning Editor at Oxford University Press, deserves thanks for her enthusiasm, expert support, and patience.
I conceived this book while in my post as Professor of Epidemiology and Public Health at the University of Newcastle upon Tyne, England. The embryonic and fetal growth stages of the idea were nourished during my sabbatical at the Department of Epidemiology of the School of Public Health at the University of North Carolina at Chapel Hill, and on my return to Newcastle. The birthplace of the book, however, was in Edinburgh University.
Secretarial support was provided chiefly by Lorna Hutchison, Carole Frazer (both Newcastle University), Betsy Seagroves (University of North Carolina), Janet Logan, and Hazel King (both Edinburgh University).
I offer grateful thanks to my wife Roma for encouraging me to finish the project, for accepting the sleep disruption caused by early morning shuffling as I slipped out of bed and into the study, and for my lack of enthusiasm for late night revelries. I have four sons, Sunil, Vijay, Anand, and Rajan, and I ask their forgiveness for sometimes being too busy and distracted to give them and their passions the attention they deserved.
R.S.B. 2001
Abbreviations xxiii
Glossary of selected terms xxv
1 What is epidemiology? The nature, scope, variables, principal measures, and designs of a biological, clinical, social, and ecological science 1
Objectives 1
1.1 The individual and the population 1
1.2 Defining epidemiology and a statement of its central idea 2
1.3 Directions in epidemiology and its uses 3
1.4 Epidemiology as a science, practice, and craft 4
1.5 The nature of epidemiological variables 6
1.6 Definition and diagnosis of disease: an illustration of the interdependence of clinical medicine and epidemiology 12
1.7 The basic tools of epidemiology: two measures of disease frequency and five study designs 15
1.8 Conclusions and seeking epidemiology’s theoretical foundations 15
Exemplar 1.1 17
Summary 19
Sample questions 20
References 21
2 The epidemiological concept of population 23
Objectives 23
2.1 The individual, the group, and the population 23
2.2 Harnessing variety in individual and group-level disease and risk factor patterns 26
2.3 Disease patterns as an outcome of individuals living in changing social groups 28
2.4 Sick populations and sick individuals 31
2.5 Individual and population-level epidemiological variables 36
2.6 Epidemiology and demography: interdependent population sciences 38
2.7 The dynamic nature of human population and the demographic and epidemiological transitions 43
2.8 Applications of the epidemiological concept of population 44
2.9 Conclusion 45
Exemplar 2.1 47
Summary 49
Sample questions 49 References 50
3 Variation in disease by time, place, and person: background and a framework for analysis of genetic and environmental effects 51 Objectives 51
3.1 Introduction to variation in disease by time, place, and person 51
3.2 Reasons for analysing disease variations: environment and genetics 52
3.3 Introducing human genetic variation and genetic epidemiology 54
3.3.1 The human genome 54
3.3.2 Genomic variation as the potential basis of human disease and population variation 56
3.3.3 Susceptibility to diseases, chronic diseases, and genetics 58
3.3.4 Mapping and manipulating the genome 61
3.3.5 Population level differences in disease and genetics: the cautionary example of race 62
3.4 Variations and associations: real or artefact? 63
3.5 Applying the real/artefact framework 68
3.6 Disease clustering and clusters in epidemiology 75
3.7 Applications of observations of disease variation 82
3.8 Epidemiological theory underpinning or arising from this chapter 83
3.9 Conclusion 83
Exemplar 3.1 84
Summary 85
Sample questions 86 References 87
4 Error, bias, and confounding in epidemiology 89 Objectives 89
4.1 Introduction to error, bias, and confounding in epidemiology 89
4.2 A classification of error and bias 93
4.2.1 Bias in the research question, theme, or hypothesis 93
4.2.2 Choice of population—selection bias, and the benefits and disbenefits of representativeness 96
4.2.3 Non-participation: non-response bias 99
4.2.4 Comparing disease patterns and risk factor–disease outcome relationships in populations which differ (context for confounding) 100
4.2.5 Measurement errors and natural variations 105
4.2.6 Regression to the mean 111
4.2.7 Analysis and interpretation of data 112
4.2.8 Publication 113
4.2.9 Judgement and action 114
4.3 A practical application of the research chronology schema of bias and error 115
4.4 Conclusion 117
Exemplar 4.1 117
Summary 120
Sample questions 121
References 123
Appendix 123
5 Cause and effect: the epidemiological approach 131
Objectives 131
5.1 Introduction: causality in science and philosophy and its relevance to epidemiology 131
5.1.1 Some philosophy 133
5.2 Epidemiological causal strategy and reasoning: the example of Semmelweis 134
5.3 Models of cause in epidemiology 136
5.3.1 Interplay of host, agent, and environment 136
5.3.2 Necessary and sufficient cause, proximal and distal cause, and the interacting component causes, models: individuals and populations 145
5.4 Susceptibility, risk/effect modification, and interaction 148
5.5 Causal graphs: introducing the directed acyclic graph 152
Exemplar 5.1 157
5.6 Guidelines (sometimes erroneously called criteria) for epidemiological reasoning on cause and effect 159
5.6.1 Comparison of epidemiological and other guidelines for causal reasoning 159
Exemplar 5.2 160
5.6.2 Application of guidelines to associations 165
5.6.3 Judging the causal basis of the association 173
5.6.4 Interpretation of data, paradigms, study design, and causal guidelines 173
5.7 Epidemiological theory illustrated by this chapter 176
5.8 Conclusion 176
Summary 178
Sample questions 178
References 180
6 Interrelated concepts in the epidemiology of disease: natural history and incubation period, time trends in populations, spectrum, iceberg, and screening 183
Objectives 183
6.1 Natural history of disease, the incubation period, and acute/chronic diseases 184
6.2 Population trends in disease 188
6.3 Spectrum of disease: a clinical concept fundamental to epidemiology 190
6.4 The unmeasured burden of disease: the metaphors of the iceberg and the pyramid 191
6.5 Screening: early diagnosis of disease or disease precursors 193
6.5.1 Introduction: definition, purposes, and ethics 193
6.5.2 Choosing what to screen for: the lasting legacy of criteria of Wilson and Jungner 195
6.5.3 Sensitivity, specificity, and predictive powers of screening tests 197
6.5.4 Setting the cut-off point for a positive screening test: introducing the relation between sensitivity and specificity and the ROC curve 201
6.5.5 Distributions of the outcomes we are screening for—explaining the relations between sensitivity and specificity 206
6.5.6 Some problems in screening: the example of blood pressure, with potential solutions 207
6.5.7 Evaluating a screening programme: biases and options 208
6.6 Applications of the concepts of natural history, spectrum, population pattern, iceberg/pyramid of disease, and screening 210
6.7 Epidemiological theory: symbiosis with clinical medicine and social sciences 213
6.8 Conclusion 213
Exemplar 6.1 213
Summary 215
Sample questions 216
References 218
Appendix 219
7 The concept of risk and fundamental measures of disease frequency: incidence and prevalence 221
Objectives 221
7.1 Introduction: risks, risk factors, and causes 221
7.2 Quantifying disease frequency, risk factors, and their relationships: issues of terminology 222
7.3 The concepts of person–time incidence rate (incidence rate) and cumulative incidence rate (or proportion) 225
7.4 Numerator: defining, diagnosing, and coding disease accurately and the challenge of using others’ data 230
7.5 Denominator: defining the population at risk and the problem of inaccurate data 235
7.6 Prevalence and prevalence proportion (but still often called rate) 237
7.7 Relationship of incidence and prevalence 243
7.8 Choice of incidence or prevalence measures 247
7.9 Presenting rates: overall (actual or commonly crude) and specific 247
7.10 Conclusion 248
Exemplar 7.1 249
Summary 253
Sample questions 253
References 255 Appendix 256
8 Summarizing, presenting, and interpreting epidemiological data 257
Objectives 257
8.1 Introduction 258
8.2 Proportional morbidity or mortality ratio 258
8.3 Adjusted rates: direct standardization and the calculation of the SMR (indirect standardization) 261
8.3.1 Direct standardization 263
8.3.2 Indirect standardization: calculation of the SMR 266
8.4 Relative measures: relative risk 268
8.5 The odds ratio 271
8.6 Measurements to assess the impact of a risk factor in at-risk groups and whole populations: attributable risk and related measures 276
8.6.1 Attributable risk: estimating benefits of changing exposure in the at-risk group 278
8.6.2 Population-attributable risk: estimating the benefits of reducing exposure in the population as a whole 281
8.7 Presentation and interpretation of epidemiological data in applied settings: health needs assessment 283
8.8 Avoidable morbidity and mortality and life years lost 285
8.9 Comparison of age-specific and summary measures of health status: perceptions and interpretation 287
8.10 Disability-adjusted life years and quality-adjusted life years 289
8.11 Number needed to treat or to prevent, and the number of events prevented in your population 291
8.12 Describing the health status of a population 292
8.13 The relationship between absolute and relative risks: a conundrum for public health action on inequalities, inequities, and disparities in health 294
8.14 The construction and development of health-status indicators 296
8.15 Handling continuous data using correlation and regression: contributions to description of health status and causal thinking 298
8.16 Conclusion 299
Exemplar 8.1 301
Summary 303
Sample questions 303
References 310
Appendices 311
9 Epidemiological study designs and principles of data analysis: a conceptually integrated suite of methods and techniques 315
Objectives 315
9.1 Introduction: interdependence of study design in epidemiology, and the importance of the base population 316
9.2 Classifications of study design: five general approaches 318
9.3 Case series: clinical and population-based register and administrative system studies 320
9.3.1 Overview 320
9.3.2 Design and development of a case series 323
9.3.3 Analysis of case series 325
9.3.4 Unique merits of and insights from case series: large-scale studies, and ecological and atomistic fallacies 326
9.4 Cross-sectional studies 327
9.4.1 Overview 327
9.4.2 Design and development of a cross-sectional study 328
9.4.3 Analysis, value, and limitations of cross-sectional studies 330
9.4.4 Cross-sectional studies in relation to the iceberg, spectrum, and natural history of disease 332
9.4.5 Cross-sectional studies in relation to case series, and potential for comparisons 332
9.5 Case–control studies 332
9.5.1 Overview 332
9.5.2 Design and analysis 333
9.5.3 Population base for case–control studies 336
9.6 Cohort study 337
9.6.1 Overview 337
9.6.2 Design, development, and analysis: prospective and retrospective cohort studies 339
9.7 Trials: population-based clinical and public health experiments 342
9.7.1 Overview 342
9.7.2 Design, development, and analysis 344
9.7.3 Natural experiments and the special case of Mendelianrandomization 347 Exemplar 9.1 349
9.8 Overlap in the conceptual basis of study designs, mixing methods, choices, birth/ age/period effects and strengths, and weaknesses 351
9.9 Ecological studies: mostly mode of analysis? 358
9.9.1 Overview 358
9.9.2 Some fallacies: ecological, atomistic, and homogeneity 360
9.10 Size of the study 361
9.11 Data analysis and interpretation in epidemiological studies: underpinning questions 362
9.11.1 Planning the analysis 362
9.11.2 Focus 364
9.11.3 Errors 364
9.11.4 Validity and qualities of variables 364
9.11.5 Generalization 366
9.11.6 Burden of disease and needs assessment 366
9.11.7 Comparability 367
9.11.8 Summarizing the contrasts 368
9.11.9 Interactions 368
9.11.10 Accounting for error, bias, and confounding 368
9.11.11 How can I assess whether my results are the play of chance? 369
9.11.12 Causality 370
9.11.13 Reviews, systematic reviews, and meta-analysis 370
9.11.14 Write up 373
9.12 Twenty questions on study design 373
9.13 Conclusion 374
Summary 376
Sample questions 377
References 383
Appendices 383
10 Epidemiology in the past, present, and the future: theory, ethics, context, and critical appraisal 389
Objectives 389
10.1 The interrelationship of theory, methods, and application: responding to criticisms of epidemiology 389
10.2 Fundamental influences on health and disease in populations 391
10.3 Theory and practice: role of epidemiology 393
10.4 From practice to theory to techniques and back 394
10.4.1 Setting priorities in health and health care: illustrative topic 1 394
10.4.2 Impact on health of local polluting industries: Teesside study of environment and health: illustrative topic 2 398
10.5 Paradigms: the evolution of sciences, including epidemiology 400
10.6 Epidemiology: forces for change 402
10.7 Scope of epidemiology and specialization 402
10.8 The context of epidemiological work: academic and service in the USA and UK 403
10.9 The practice of epidemiology in public health 404
10.10 Ethical basis and proper conduct of epidemiology: the need for a code 405
10.10.1 The tobacco industry manipulates researchers including epidemiologists 405
10.10.2 Authorship: manipulation by researchers 406
10.10.3 Ethnicity and race: purpose and direction in epidemiology 407
10.11 Ethical guidelines for epidemiology and the clash of informed consent, data privacy, and the pursuit of epidemiology 409
10.12 Critical appraisal in epidemiology: separating fact fromerror and fallacy 410
10.12.1 Some fallacies 412
10.12.2 The nature of critical appraisal 414
10.13 Some questions relevant to the appraisal of epidemiological research 414
10.14 Historical landmarks and the emergence of modern epidemiology 416
Exemplar 10.1 417
10.15 A reflection on the future of epidemiology 419
Summary 423
Sample question 424
References 424
Appendix 1 Possible curriculum for taught courses for undergraduate and postgraduate students and in continuing education 427
Appendix 2 Further reading and useful websites 429
Index 431
Abbreviations
AIDS Acquired Immune Deficiency Syndrome
AR Attributable risk
AUC Area under the curve
BCG Bacillus Calmette–Guérin
BMI Body mass index
BMJ British Medical Journal
BSE Bovine spongiform encephalopathy
CDC Centers for Disease Control and Prevention
CHD Coronary heart disease
CHI Community Health Index
CI Confidence intervals
CJD Creutzfeldt-Jakob disease
CVD Cardiovascular disease
DAG Directed acyclic graph
DALY Disability-adjusted life years
DNA Deoxyribonucleic acid
ECG Electrocardiogram
ECT Electroconvulsive therapy
ERSPC European Randomized Study of Screening for Prostate Cancer
EQUATOR Enhancing the QUAlity and Transparency Of health Research
GBD Global Burden of Disease
GIS Geographical information systems
GNP Gross National Product
GRACE Global Registry of Acute Coronary Events
GWAS Genome-wide association study
HALY Health-adjusted life year
HDL High-density lipoprotein
HEA Health Education Authority
HIV Human Immunodeficiency Virus
ICD International Classification of Diseases
ICMJE International Committee of Medical Journal Editors
IEA International Epidemiological Association
LDL Low-density lipoprotein
LVH Left ventricular hypertrophy
MI Myocardial infarction
MINAP Myocardial Ischaemia National Audit Project
MMR Measles, mumps, and rubella
NEPP Number of events prevented in a population
NHANES National Health and Nutrition
Examination Surveys
NHS National Health Service
NIH National Institutes for Health
NNT Number needed to treat
NNP Number needed to prevent
NICE National Institute for Clinical and Public Health Excellence
OR Odds ratio
PAHO Pan American Health Organization
PAR Population-attributable risk
PI Physical inactivity
PM Proportional mortality
PMR Proportional morbidity or mortality ratio
PODOSA Prevention of diabetes and obesity in South Asians
PPOD Population pattern of disease
PSA Prostate specific antigen
PYLL Potential years of life lost
QALY Quality-adjusted life years
RCT Randomized controlled trial
RNA Ribonucleic acid
ROC Receiver operating characteristic
RR Relative risk
SEP Socio-economic position
SHELS Scottish Health and Ethnicity Linkage Study
SIDS Sudden infant death syndrome
SMR Standardized mortality ratio
SNP Single nucleotide polymorphism
TB Tuberculosis
UNESCO United Nations Educational, Scientific and Cultural Organization
UK United Kingdom
USA United States of America
US United States (of America)
UV Ultraviolet
WCEC World Council on Epidemiology and Causation
WHO World Health Organization
YOLL Years of life lost
Glossary of selected terms
The glossary focuses on clinical terminology, and terminology relating to directed acyclic graphs (introduced in the third edition), and excludes many terms and concepts considered in the main text, which can be accessed via the index.
Acute An adjective commonly applied to diseases that have a short time course.
Adenocarcinoma A cancer of mucosal cells.
Aetiology/aetiological Causation/about causation.
AIDS Acquired immune deficiency syndrome, the serious multisystem disease resulting from infection by the human immunodeficiency virus (HIV).
Allele see Gene.
Ancestor In DAG (see entry) terminology all preceding factors in the causal line that are linked to the exposure under study.
Anorexia Impaired appetite for food which can lead to serious illness; when caused by psychological factors it is called anorexia nervosa.
Angina see Rose angina.
Arrhythmia An abnormal rhythm—usually applied to the heart.
Association A link, connection, or relationship, between risk factors, between diseases, and most usually between risk factors and diseases.
Asthma A respiratory disease characterized by difficulty in breathing caused by narrowing of the airways (which is reversible).
Atherosclerosis/atherosclerotic The process of deposition of lipids and other materials in the walls of arteries leads to their narrowing and to inflammation. Such an artery is atherosclerotic (also known as arteriosclerosis/arteriosclerotic).
Atrial fibrillation An irregular heart beat resulting from very rapid and irregular contractions of the atrium (one of the chambers of the heart).
Attenuation Reduction of the measured effect of a cause in epidemiology. In microbiology it means that the virulence of microbes is reduced through processes that attenuate the effects (e.g. in the production of vaccines).
Autopsy see Necropsy.
Autosomal A genetic disorder relating to any chromosome except the sex (X and Y) chromosomes.
Backdoor path A non-causal path arising from an association with a third factor between a putative cause and outcome. In epidemiology, this is the effect of the confounding variable. In a DAG the arrow for this path points to the exposure.
Blocked/Blocking A causal path that is closed off as a result of the third factor (confounder).
Blood pressure Usually refers to the pressure in the systemic arteries (not veins and pulmonary arteries), as measured by sphygmomanometer (see below).
Brucellosis An infection caused by Brucella microorganisms, characterized by recurrent fevers.
Capture–recapture method A technique whereby two or more incomplete data sets are used to estimate the true size of the population of interest. Usually in epidemiology, the method is applied to populations that are hard to count (e.g. the homeless).
Cancer see Neoplasm.
Carcinogen(ic) A substance that increases the risk of developing a cancer (an adjective describing such a substance).
Case A person with the disease or problem under investigation.
Caseous An adjective applied to cheese-like lesions produced in response to inflammation (e.g. in tuberculosis).
Cataract An opacity in the lens of the eye.
Causal diagram A general term for a variety of approaches to showing the relationships between variables in graphic form. Within epidemiology these diagrams set out the relationship between exposure variables and outcomes, in relation to other variables such as confounders. One formal causal diagram is the directed acyclic graph (DAG).
Causal graph see Causal diagram (synonym).
Causation The term is commonly used in epidemiology, public health, and medicine as an alternative to aetiology.
Cause Something which has an effect; in the case of epidemiology this effect being (primarily) a change in the frequency of risk factors or adverse health outcomes in populations.
Causality The relationship between causes and effects.
CAD or coronary artery disease see Coronary heart disease
Cholesterol A lipid (fatty substance) that is essential to many bodily functions that is transported in the blood via lipoproteins. Cholesterol and other lipids carried by low/very low-density lipoproteins (LDL/VLDL) are a risk factor for cardiovascular diseases, while those carried by high-density lipoproteins (HDL) may be protective.
Child In DAG terminology—the immediate descendant of the variable.
Chromosome see Gene
Chronic An adjective commonly applied to diseases that have a long-lasting time course, and usually applied to non-toxic and non-infectious diseases.
Chronic bronchitis A lung disease characterized by production of excess sputum, wheezing, shortness of breath, and eventual respiratory failure.
Circadian A rhythm with a cycle lasting about 24 hours, corresponding to each day.
Collapsibility The situation where measure of association remains unchanged despite omission of variables from the model and where stratified and un-stratified results are the same. Collapsibility is not the same as confounding but a more general concept including both confounding and M-bias.
Collider A term used in DAGs for a variable where both the exposure and the outcome have an arrow (path) leading to it. In other words, the exposure and outcome, together, are the causes (parents) of the collider.
Competing causes (or competing risks) A concept where alternative causes of disease, or more usually causes of death, are in competition with each other; for example, in African Caribbean populations one explanation of the comparatively low CHD rates is that the atherosclerotic process kills people from stroke. If stroke were to be controlled, it may be that CHD would become more common.
Collider stratification bias see M-bias.
Community medicine see Public health
Conditioning bias see M-bias.
Confidence interval The interval, with a given probability (e.g. 95%), within which is contained the population value of a summary measure such as a mean from a sample.
Confounding The distortion of the measure of an association between two variables by other (confounding) factors that influence both the variables under study.
Contagion/contagious The infectious diseases that may be transmitted from person-to-person.
Congenital A health problem present at birth.
Cooling tower A term that describes a structure, sometimes including evaporative condensers, designed to extract heat from a liquid—usually water—before its recirculation.
Coronary heart disease/coronary artery disease A group of diseases resulting from reduced blood supply to the heart, most often caused by narrowing or blockage of the coronary arteries that provide the blood supply to the heart.
Correlation analysis The extent that variables are associated with each other. The word is usually applied to continuous variables. The association can be measured by a statistical technique giving the correlation coefficient which varies from –1 (inverse correlation) to +1 (positive correlation).
Cot death A synonym for the sudden infant death syndrome, which is unexplained death in infancy.
Crohn’s disease A disorder usually of the lower part of the small intestine, characterized by inflammation.
Cutaneous Associated with the skin.
D-connected/connection A path that is open.
D-separation or D-separated A path that is closed, either through confounding or M-bias or both. So it consolidates the ways in which a causal path con be blocked.
DAG Acronym for directed acyclic graph; see Causal diagram and Directed acyclic graph.
Deep vein thrombosis Clotting of blood in the deep veins usually of the calves, thigh, and pelvis. The clots may lead to a pulmonary embolus.
Degenerative An adjective applied to disease thought to be resulting from deterioration of tissues over time (i.e. with age).
Demographic transition The change in the age structure of the population after death rates and fertility rates decline, with a comparatively older population being the result.
Demography The scientific study of population, particularly the factors that determine its size and shape, including birth and death.
Depression A serious clinical illness with a characteristic set of symptoms and behaviours that, in its most severe form, may lead to suicide.
Descendants The variables arising from a postulated effect (in DAG terminology).
Developmental origins of disease see Fetal origins of disease hypothesis
Diabetes mellitus A disease characterized by high levels of glucose in the blood caused by either lack or ineffectiveness of the hormone insulin.
Diethylstilboestrol A medication containing the hormone oestrogen.
Differential diagnosis A preliminary list of the most likely diseases in a patient, to be confirmed by further investigation.
Directed acyclic graph (DAG) A formal mathematical method for producing a postulated causal diagram. The diagram shows one-way relationships, hence it is called acyclic, and the direction of relationships is shown by arrows.
Disease A bodily dysfunction, usually one that can be described by a diagnostic label. (For simplicity, this book concentrates on discussing diseases and uses this word loosely when other words describing other health problems would be more specific, e.g. death, disability, illness, sickness, etc.)
Distribution The frequency with which each value (or category) of a variable occurs in the study population. The distribution of many variables takes on a characteristic shape. Also see Normal distribution.
Dizygotic Usually refers to non-identical twins who have shared the womb, but result from two fertilized eggs (see Monozygotic).
Down’s syndrome A congenital genetic disorder, leading to mental retardation and a characteristic face, caused by the presence of three chromosomes instead of two at the site of the twenty-first chromosome.
Edge The line that is the basis of the arrow in a DAG.
Effect modifier A factor that alters the causal impact of another factor, the classical example being that smoking cigarettes greatly modifies (increases) the risk of lung cancer after exposure to asbestos and vice versa.
Elective In health care, a procedure done at a time chosen for its convenience, as opposed to being dictated by an emergency (e.g. elective surgery).
Electrocardiogram (ECG) A recording of the electrical activity of the heart made by an instrument called an electrocardiograph.
Emphysema When applied to the lungs (pulmonary emphysema) a condition caused by destruction of lung tissue with gaseous distension.
Endogenous selection see M-bias.
Environment A broad concept in epidemiology, sometimes meaning everything except genetic and biological factors, and sometimes more qualified and narrow (e.g. physical environment).
Epidemiological transition The change in disease patterns that accompanies the demographic transition, with both transitions usually following economic, educational, or social improvements.
Epidemiology See text, but in short, the study of the pattern of diseases and their causes in populations.
Epilepsy A disease of the brain characterized by temporary, involuntary, altered states of consciousness, including fits (convulsions).
Equity/inequity An equality that is required by our rules on fairness and justice. An inequality that is unfair or unjust is called an inequity. Inequity is a subset of inequality, as not all inequalities are unfair.
Estimability see Identification.
Ethnicity The group you belong to, or are perceived to belong to, or a mix of both, as a result of cultural factors such as dress, religion, diet, name, ancestry, and physical appearance (e.g. skin colour).
Exchangeability A concept indicating that two groups are the same in terms of their probability of an event (e.g. on outcome). In other words, the groups are alike.
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his illegal barter with the Indians to the great consternation of the Pilgrims.
Beaver, otter and valuable deer skins found their way in great quantities to the new truck house, while Merrymount became “a sort of a drunkard’s resort and gambling hall” and worse. The jolly host of Merrymount, revelling in “riotous prodigality,” had a great Maypole erected for the entertainment of visiting factors who wanted to dance and frisk with pleasurable Indian maids. Fishing and trading vessels along the coast much preferred to do business with the open-handed Morton rather than the close-fisted Pilgrims. Therefore, trading goods were as easily come by at Merrymount as were beaver skins. Everyone was quite happy about the situation except the fathers at Plymouth.
Morton had even discovered their profitable new trading grounds in Maine. The first year the Pilgrims extended their fur trading operations to Maine, in 1625, they gathered in 700 pounds of beaver besides some other furs on the Kennebec, mostly in exchange for the corn they had by then learned to grow at Plymouth. But now Morton was outbidding them with his more attractive trading goods and getting nearly everything of value in that vicinity too. The very existence of the Plymouth Colony seemed to be at stake.
Complaints, cajolings and threats were of no avail. Morton, the lawyer, was too clever. Something had to be done and force was the final resort. Captain Standish was sent with some soldiers to arrest the obnoxious neighbor on the charge that he was violating the royal proclamation prohibiting trade in guns and powder.
“Captain Shrimp,” as Morton contemptuously called Standish, succeeded in his mission only after a fight. He captured Morton and took him to Plymouth. There he tried his best to have the erstwhile “host of Merrymount” hanged. But in the end Morton’s only punishment was to be shipped back to England.
As it came about, the Wessagusset area was no sooner eliminated as a competitor for the beaver trade than the Pilgrims were faced with new and much more formidable rivals in the same neighborhood. These were the Puritans, the advance guard of whom
arrived at Salem under Captain John Endicott in 1628. As recruits of the great Massachusetts Bay Company they were soon coming by the thousands. Under the leadership of Governor John Winthrop they overran the Boston Bay region. And there wasn’t much the fathers at Plymouth could do about these new neighbors except to offer religious advice on the relative merits of Brownism and Separation.
The local competition for beaver now being almost overwhelming, the Pilgrims found it necessary to go more and more afield to meet their required quota of pelts. Excursions north to the Kennebec in Maine were the most fruitful, not only because there was much fine fur to be had from the natives there but because trading goods on occasion could be obtained from fishing and trading ships off that coast. Once, too, in exchange for corn the Pilgrims picked up four hundred pounds worth of trucking stuff from some Englishmen who were abandoning their plantation on Monhegan Island. And they acquired an additional stock of truck from a French ship which had been wrecked at Sagadahoc.
To the south along the coast they never met with much success. When they had first gone out to Narragansett Bay on a trading voyage in 1623, for instance, the Indians there disdained their meagre offerings. The Narragansett were much too happy with the goods being furnished by the Dutch traders from Manhattan. Both the Narragansett and their neighbors, the Pequot, held on to their furs for the Hollanders, and they were powerful enough to get away with it. They had Dutch guns and powder, as well as the Hollanders themselves if need be, to back them up. In fact, by 1625 the Dutch had a fortified trading post on an island in Narragansett Bay and two similar forts near-by on the mainland.
Governor Bradford, in exasperation, gave out a warning that all the region along the coast to the southeast of Plymouth was English territory. He as much as demanded that the New Netherlanders stop trading there. But it was useless. The Dutchmen simply ignored the
warnings. However they did offer to enter into direct trading relations themselves with Plymouth and sent a mission there in 1627, with sugar, linen and other goods, to talk it over.
This mission was headed up by Isaack de Rasieres, the chief trader as well as the Secretary of New Netherland. Rasieres appears to have had an ulterior motive in making the visit. He brought along a stock of wampum, the strings of highly polished shell-beads that the Dutchmen had been accustomed to getting from the Narragansett and the Pequot. With wampum, Rasieres pointed out to the Pilgrims, he had been doing a great business among Indians who didn’t have the means of manufacturing it, especially along the Hudson River where Henry Hudson himself had first discovered strings of shellbeads circulating as a kind of money.
The Hollander cannily suggested that wampum might be used to just as great advantage on the Kennebec by the Pilgrims. No doubt, by this means, he hoped to direct their attention more to Maine and away from the Dutch trading preserves in Long Island Sound and the Narragansett country. Offering to sell wampum to the Englishmen at a fair price, probably he hoped also to keep them from dealing direct for it with the Narragansett and the Pequot.
Although the Pilgrim fathers were a bit suspicious of Rasieres’ motives, nevertheless they did find the shell-money to be most “vendable” on the Kennebec. So much so, in fact, that in a very short time with the aid of this medium of exchange they were enabled to cut off the fur trade of that region from the fishermen and other independent traders who had been accustomed to barter there. In the meantime they also developed their own sources of wampum among some of the Massachusetts coastal tribes who, as it turned out, also possessed the means of manufacturing it.
The Pilgrims secured their rights up the Kennebec River by obtaining a patent to a strip of land fifteen miles wide on both sides of that river They built a trucking house at Cushenoc (Augusta) which they kept stocked with coats, shirts, rugs, blankets, corn, biscuit, peas, prunes, and other supplies. With the help of wampum for exchange they drove a brisk trade among the Abnaki Indians in
those parts. In 1630 they extended their operations even farther north, setting up a trading post on the Penobscot River at Pentagoet, now Castine, Maine.
The Penobscot trading post originated as a private venture for which the Pilgrim, Isaac Allerton, along with some partners in England promoted a patent. Allerton, once a London tailor, had risen in prominence at Plymouth in New England to stand second only to the governor. It would appear that he and his overseas partners shared the rights to trade in the territory north of the Kennebec with other leading Pilgrims in return for the loan of wampum, shallops, supplies and servants.
But the partnership accounts became jumbled, because of Allerton’s financial deceits according to the Pilgrims. And Edward Ashley, the “profane young man” sent out from England as factor, didn’t help things by his personal conduct.
Actually, young Ashley did well enough in the bartering department, acquiring over a thousand pounds of beaver and otter the first season. However he seems to have gone native in the most offensive sense to the Pilgrim fathers, living “naked” among the savages and committing “uncleannes with Indean women.” Also, it was discovered, he was trading shot and powder with the savages and not even accounting for the profits from this unholy trade.
Such behavior, of course, could not be countenanced. Ashley was arrested and shipped back to England by the Plymouth partners, while they themselves took over complete operating control of the enterprise.
All of which was not to Isaac Allerton’s liking. Deserting the partnership, he retaliated by setting himself up as a competitor in Maine. In 1633 he settled some “base fellows” in a new trading post at the mouth of the Machias River, close by the present Canadian border. There he did his best to cut off Plymouth’s commerce with the more northerly tribes who were then taking their furs to the Penobscot and Kennebec trucking houses.
The Pilgrims had other competitors in Maine also. In 1630 a trading post was established at Pemaquid Point under a patent obtained by two English merchants for twelve thousand acres between the Damariscotta and Muscongus Rivers. About the same time John Oldham and Richard Vines obtained a grant at the mouth of the Saco River, which remained a most profitable fur trading center for some years.
Then there was the Laconia Company, organized by Sir Ferdinando Gorges and Captain John Mason to put new life in their Piscataqua colony. Laconia, described as a vast hinterland area of rivers and lakes, was an extension of their original patent to the “Province of Maine.” Gorges and Mason now planned to send cargoes of Indian trucking goods up the Piscataqua River into Laconia, to Lake Champlain, to be bartered for peltries. Thus they hoped to compete with the Dutch and the French for the hinterland trade.
Captain Walter Neale, as governor of the Laconia Company, and Ambrose Gibbons, as factor, did very well from 1630 to 1633, establishing several trading posts on the Piscataqua River But they never reached Lake Champlain. Maybe they had it confused with Lake Winnepesaukee. In any case, as they discovered, the rivers of Maine flowed from the north, not from the west, and they couldn’t penetrate deep enough into the interior to tap the hinterland trade of the Dutch and the French.
Probably the most dangerous of the Pilgrims’ rivals in Maine about this time was the trading post on Richmond’s Island off Cape Elizabeth. Thomas Morton, the jolly host of Merrymount, had traded here as early as 1627. After his banishment from America one of his most roguish associates, Walter Bagnall, took over the island and is said to have gained 1,000 pounds sterling from his trade in a period of three years. Then this “wicked fellow” was murdered by the natives.
Bagnall’s successor, John Winter, also did a flourishing business on Richmond’s Island as factor for some English merchants, employing some sixty men at one time in both fishing and fur trading
activities. The records indicate that he was about as unscrupulous in his dealings as was his predecessor, cheating and otherwise mistreating the Indians at every turn. He charged them at the rate of thirty-three pounds for a hogshead of brandy which cost him seven. For powder which cost him twenty pence per pound he raised the rate to three shillings in trade. But the natives preferred his goods.
In spite of such competition, however, Maine became the Pilgrims’ chief source of furs. In one period of two years the trading post at Cushenoc alone is known to have gathered in more than 7,000 pounds of beaver. The route of the beaver-laden shallops from the Maine coast was indeed Plymouth’s life-line. Wampum played no small role in the success of this extended operation which was so vital to the colony’s existence, although the tight control exercised over the fur trade by the Pilgrim fathers was the main factor.
Always the peltry traffic had been invested in certain leaders at Plymouth who managed the whole trade in the interests of the communalty in order that the Pilgrims’ debt to the merchants in England might be paid. In 1627 William Bradford, Isaac Allerton, Edward Winslow, Miles Standish and a few others in partnership undertook to assume this entire indebtedness. In return, these “undertakers” were to enjoy any and all profits from the traffic in peltries for six years. They were empowered to do what they pleased with all furs and trucking goods in the common store, and they alone were to use the colony’s trading posts, truck houses and shallops.
From that time forward the Plymouth Plantation enjoyed real economic success. In 1628 furs to the value of 659 English pounds were exported in one cargo. Although no total figures are available for 1628 to 1630, a very large amount of peltry went to England according to Governor Bradford. From 1631 to 1636 over 12,000 pounds of beaver and 1,000 pounds of otter were shipped, most of which brought 20 shillings a pound and none less than 14 shillings. Bradford said that the beaver alone during these years brought 10,000 pounds sterling and that the otter was more than sufficient to pay all costs of transport and auction. Considering the size of the plantation this was indeed big business.
In spite of the financial bungling of the partnership’s agents in England, and even outright irregularities there, the debt was paid off. Additional profits from the sale of beaver skins bought many other things the Pilgrims needed, and in the end it was the earnings from the fur trade that provided the foundation for Plymouth’s next economic development, cattle farming.
In the meantime the Pilgrims’ profitable trade in furs on the Maine coast and on rivers leading to the interior was jealously regarded by the French who, after all, considered that territory within their own limits. And neighboring French traders were doing something about it.
A Border Fixed on the Coast of Maine
THE planting of trading posts farther and farther up the coast of Maine by the New Englanders did not go unchallenged by the French. Although in English eyes it had been well established ever since Captain Argall’s raids in the Bay of Fundy that English claims to the coast now extended well above the 45th parallel, even to the St. Lawrence valley, no such admission had ever been made by the French. There were always French traders in Acadia (Nova Scotia) who envisaged their preserves as extending well down the coast of Maine and who intended to resist further encroachments in those parts.
In fact, as it came about, these fur traders protected and maintained the French claim to much of this coastal region during an incident of the Thirty Years’ War which put most of New France under the English flag for three years, from 1629 to 1632.
Prior to this episode the affairs of New France had reached a turning point. In 1627 Samuel de Champlain’s difficulties became acute. That year the Iroquois, his irreconcilable enemies, renewed their bloody savagery in the interior, while along the coast the English were closing off the very entrance to the valley of the St. Lawrence with their plantations and trading posts and their increased shipping.
Although Champlain’s company had a good season in furs, collecting 22,000 that summer which could be sold in France at ten francs each, the expense of operation had made the outlook none too bright relatively for the shareholders who expected huge returns. Besides the salaries of the Viceroy in France and of Champlain as his lieutenant governing the colony, interpreters now cost as much as a thousand francs a season and sailors six hundred, while factors
and other servants came correspondingly high. And there were other increased expenses that bid fair to cut deep into profits.
As a result the directors of the company made things difficult for their governor on the St. Lawrence. They particularly balked at his urgent request for extra funds to strengthen the fortifications of Quebec, even though only a wooden palisade and a few small cannon protected the French trading post.
Champlain, with characteristic fortitude, made the best of his difficulties. But he was most uneasy about the English at his doorway And well he might be!
As early as 1620 Sir George Calvert, English Baron of Baltimore, had adventured a plantation in Newfoundland. Three years later King James granted him quasi-regal proprietorship of the southeastern peninsula of that island between Trinity and Placentia Bays. In 1627 he came out with his family, built a fine house at Ferryland, and showed every intention of establishing a great fur trading and fishing colony in the form of a British dominion at this strategic gateway to New France.
Still more alarming were the activities of a Scotsman, Sir William Alexander, who had been stirred by accounts of the founding of New Spain, New France, New Netherland and New England to attempt the founding of a New Scotland. In 1621, with complete disregard for French claims, the English king had granted to him all the vast peninsula between the Atlantic Ocean and the Gulf of St. Lawrence! Today that would be Nova Scotia, New Brunswick and part of Quebec Province. This princely domain was to be divided into feudal-type Scotch baronies complete with a hierarchy of hereditary titles.
Alexander’s attempts to establish permanent trading posts in New Scotland, or Nova Scotia, were failures until 1628. That year seventy-two of his colonists managed to set up for business at Poutrincourt’s old quarters at Port Royal in the Bay of Fundy. There they found themselves to be the near neighbors of a young French fur trader at Cape Sable named Charles de la Tour who was under the impression that the country belonged to him.
Monsieur La Tour had inherited his rights from Saint Just after that pioneer finally returned to France in 1623. Since then he had been living the independent life of a wilderness lord with French retainers and aboriginal subjects, and he had no intention of yielding his estate or fur-trading privileges to anyone, much less foreigners.
La Tour did not immediately attempt to oust Alexander’s Scots from Port Royal as he was on much the weaker side at the time, but he did stand firm and unsubmissive at Cape Sable and he resisted all efforts of the English to bring him over to their side. This was not easy, for nearly all of New France was soon in the hands of the enemy.
In 1628 a French colonizing fleet sent out by the Company of New France under the aegis of Cardinal de Richelieu was captured by privateers boldly operating in the Gulf of St. Lawrence with English letters of marque. In another year Champlain’s worst fears were realized when this same fleet of privateers, under the leadership of the Kirke brothers, appeared before Quebec in force enough to insure its capitulation.
Champlain himself was taken prisoner at Quebec. But under the terms of the surrender he marched out of the fort with his arms and all of his own furs, as did other head men. The factors, servants and workmen were allowed to take but one beaver skin each before the English took over the factory. Then, after running up the English flag, the Kirkes were said to have driven an immediate trade with Indians and others remaining about the fort for some 2,000 skins in addition to those they found in the storehouse. They even ended up with the pelts they had allowed the starving garrison under the terms of surrender, taking them in trade for food.
Sir William Alexander, uniting his interests with the Kirkes to form the Scottish and English Company for the peltry trade of New France, sent out a second fleet to Port Royal in 1629 under the command of his son. When the fleet arrived nearly half the people who had been left at Port Royal the year before were found to have died from one cause or another. But young Alexander relieved the survivors, strengthened the fort, and put new life into the colony. His
vessels managed to acquire a satisfactory number of pelts in the Bay of Fundy during that summer. He also captured a French ship. And as further proof of the substance of New Scotland he took back on his return voyage an Indian chief of the region who wanted to conclude an alliance with the English king.
In the meantime it developed that shortly before Champlain’s capitulation a treaty of peace had been signed between England and France. But, although King Charles agreed very soon to restore Quebec to the French under the terms of the treaty, the negotiations dragged on for three years. During that time the Scottish Company continued to drive a great trade for pelts on the St. Lawrence and elsewhere, while the French pressed not only their claims to furs appropriated from stores at Quebec after the actual signing of the peace treaty but insisted also upon the return of all of New France. That included Acadia of course and the evacuation of the fort at Port Royal.
At first, Charles was not disposed to disturb Alexander in his occupation of “Nova Scotia.” In fact he encouraged the Scot to further efforts in that direction However, he needed gold at the time much more than he needed another Scotland, so he was not reluctant to do a little bargaining. When his wife’s brother, the French king, agreed to pay a long overdue and substantial dowry, Charles agreed to return Port Royal to the French. Sir William’s dream was thereupon finished. The Scots demolished their fort, and young Alexander surrendered Port Royal to the Chevalier Isaac de Razilly who had been sent out by the Company of New France as governor of all Acadia.
This was a tremendous victory for Charles de la Tour and his French fur traders, who had not only stubbornly maintained themselves during the incident of the Scottish occupation of Acadia, but had managed harassments intended to hinder further encroachments up the coast of Maine by the Englishmen in those more southerly parts.
In 1631 the Frenchmen had paid a visit to the Pilgrims’ most northerly outpost at Pentagoet on the Penobscot. Taking advantage
of the absence of the factor and most of his company, they were able to surprise a few “simple” servants by a ruse. First, pretending they had “newly come from the sea” and that their vessel was in need of repairs, a “false Scot” among them fell to admiring the Englishmen’s muskets. Then, talking the servants into letting them examine the guns, they gained possession of the Englishmen’s weapons and promptly made away with some four or five hundred pounds worth of Pilgrim goods, including three hundredweight of beaver.
In time, however, after the ousting of the Scots from the Bay of Fundy, the Frenchmen were able to do more than simply harass the Englishmen on the coast of Maine. The truck house at Machias in Maine, built by Isaac Allerton in 1633, was even more of a challenge than the Penobscot post had been. Allerton had no sooner settled his traders there than “La Tour, governor of the French in those parts, making claim to the place, came to displant them, and finding resistance, killed two of the men and carried away the other three and the goods.”
Allerton himself later went to Port Royal to protest. But he was told by La Tour that “he had authority from the king of France, who challenged all from Cape Sable to Cape Cod, wishing them to take notice and to certify the rest of the English, that, if they traded to the east of Pemaquid, he would make prize of them.” When Allerton asked to see Monsieur La Tour’s commission the Frenchman replied that “his sword was commission sufficient, where he had strength to overcome; where that wanted, he would show his commission.”
And the French made good on their challenge, for in 1635 Monsieur Charles d’Aunay, one of Governor Razilly’s lieutenants, came in a man-of-war to the Pilgrims’ trading post on the Penobscot. By a show of this force he took possession of the trucking house in the name of the King of France. Making an inventory of the goods he found there at prices he set himself, d’Aunay “made no payment for them, but told them in convenient time he would do it if they came for it. For the house and fortification etc. he would not allow nor account anything, saying that they which build on another man’s ground do forfeit the same.” Then he put the English traders in a shallop and sent them back to Plymouth.
The Pilgrim fathers were enough upset about the loss of their goods and trading post to do something violent about it. With the approval of the Massachusetts Bay Colony, but without much in the way of material assistance from their Puritan brethren, they hired “a fair ship of above 300 tun well fitted with ordnance” to retake Pentagoet. This ship was under the command of a Captain Girling. If the captain was successful he was to have 700 pounds of beaver; if not, nothing. Along with him went Captain Miles Standish and twenty Plymouth men in their own bark to resettle their trading post after Girling had driven out the French.
It would appear, however, that the French must have had notice of the impending attack. So firmly were they entrenched behind earthworks that Captain Girling’s gunfire could not dislodge them and all the big ship’s powder was exhausted without effect. No landing was attempted. Standish, frustrated, returned to Plymouth in his bark, and Captain Girling went his own way.
The French now remained in permanent possession of what was soon known as the “Mission of Pentagoet.” And so the line was finally drawn as a practical matter between the English and the French on the Maine coast, although England continued to maintain officially that it was more northerly, at the 45th parallel.
Mutual distrust, born of the religious differences between English Puritan and French Catholic, often erupted in charges and countercharges that sometimes threatened security on either side of the line. In fact, fear of possible French aggression was in part responsible for the eventual formation of the New England Confederation in 1643. On the other side of the picture however, there was much guarded trade between Frenchman and Englishman as it suited their pocketbooks. As a matter of fact the Puritans actually traded with the French conquerors of Pentagoet shortly after d’Aunay captured the Pilgrim trading post.
But, even so, this coastal border created by the rival fur traders of these two nations was maintained with only minor variations for years, even after competition for the beaver trade was no longer the reason for its existence.
Acadian fur traders and the rivers of Maine and Massachusetts that ran in the wrong direction had effectively contained the English on the northern coasts. The St. Lawrence River route to the interior, to the great lake country that teemed with fur-bearers, remained safe to the French traders, who resumed their profitable westward penetration of the hinterland.
It remained to be seen, however, if the French could prevent the Dutch of New Netherland and the English of Virginia from draining off this hinterland trade. The Connecticut, Hudson, Delaware, and Susquehanna Rivers all ran in the right direction to tap it.
The Bay of Virginia
WHEN Sir George Calvert, Baron of Baltimore, founded his province of “Avalon” in Newfoundland he was partly motivated by religious urgings. Being a Catholic convert he earnestly wanted to provide a haven in the new world for the persecuted Catholics of England. But this court favorite, who as the son of a humble Yorkshire farmer had figuratively pulled himself up by his own bootstraps to ennoblement, was also a profit-minded promoter. He expected a rich return on his newly gained proprietorship.
However, after investing many thousands of pounds at Ferryland, Lord Baltimore had little to show in the way of profits. The future, too, looked as bleak as the cold Newfoundland winters, as barren as the sterile soil. True, there were fish to be had for the taking—weather permitting. But the trade in pelts was only fair, for the more profitable fur frontier had long since passed into the interior of Canada. And agricultural pursuits, the traditional support of English feudal manors, were almost impossible due to the inclemencies of the climate.
The Lord Proprietor of Avalon was soon looking southward, his eyes resting appraisingly on Virginia. There, the warmer weather and richer soil seemed to offer more appropriate conditions for the kind of province he had in mind. There, in the lush tidewater of the Chesapeake Bay country, he could hope for a better trade in pelts too.
As for possible profits from Newfoundland cod, he would gladly consign them to those hearty fishermen who by nature were better able to cope with the rigors of the northern climate!
So it came about that in 1629 his lordship wrote to King Charles asking that a precinct of land in Virginia be granted to him with the same quasi-regal privileges he enjoyed under his Newfoundland
patent. Then, without waiting for a reply, he embarked for Jamestown where he let it be known that he was looking for a new plantation site, and forthwith began cruising about Chesapeake Bay with the air of a man who was confident that the king would approve his plans.
This was anything but politic. No Virginians, Anglican or Puritan, wanted a Catholic in their midst. They didn’t want one even as a neighbor in their great bay, and especially such an influential convert as Baltimore. Who knew what Romish plot he might be promoting?
But even more alarming was the threat that this ambitious baron posed to their fur trade in the Chesapeake.
The tidewater had come to represent a treasure house of pelts to the merchants and factors on the James River, at Jamestown and at Kecoughtan, and to scores of traders plying their shallops and pinnaces in the bay. On the York, the Rappahannock and the Potomac Rivers these traders regularly visited the villages of the Indians, their shallops returning to the James River plantations laden with skins. And across the bay on the Eastern Shore there was still another lively fur trading center at Accomac where beaver and muskrat abounded in the near-by streams and marshes.
The most profitable branch of this trade was that in the Potomac River, where the Spaniards of the sixteenth century had been the first of record to barter with the natives for pelts and hides. Captain Samuel Argall, who succeeded Captain John Smith in 1609 as the colony’s main support for trade with the Indians, made several early visits to the villages of the Patowomecks there. For hatchets and hoes, no doubt, he took good quantities of beaver, otter, raccoon and deer skins as well as the corn that was so badly needed at Jamestown. In 1612 he really whetted the Patowomecks’ appetite for English goods when he produced a copper kettle for their chief. Many English traders, with and without permits, followed Captain Argall into the Potomac River, but especially members of the Virginia
Council who more or less controlled the Chesapeake trade as a perquisite of office.
The Eastern Shore was opened up for trade in 1619 by Ensign Thomas Savage who was one of the earliest professional factors in the Chesapeake tidewater. He had spent several years as a youthful hostage with Chief Wahunsonacock of the Powhatan Confederacy after having been left with this old Powhatan in 1608 by Captain Christopher Newport. During that time young Savage became a favorite of the natives and learned much about the dialects and customs of the tribes in the tidewater country. Later, when his services as an interpreter came to be in demand by Virginia officials, he was commissioned an “Aunchient,” or ensign, on the staff of the Master of Ordnance at Jamestown. It was in this capacity on expeditions of discovery in the Potomac and Patuxent Rivers and on the Eastern Shore that he began bartering on his own account.
Like Captain John Smith before him, Ensign Savage learned of the great trade in furs being had by the Frenchmen on the “backside of Virginia.” In 1621 his account of their deeper penetration of the hinterland reached the Virginia Company Council in London. At the same time the company received reports of Dutch traders operating along the Virginia coasts. A detailed account of this competition was rendered by Thomas Dermer, the New England fur trader, who in addition to having paid the visit to Jamestown went into the Delaware and Hudson Rivers. He reported that in both those rivers he had “found divers ships of Amsterdam and Horne who yearly had there a great and rich trade in Furrs.”
The Dutch encroachments prompted a special letter, too, from the governor and his council at Jamestown. They urged the company to undertake “so certaine and gainefull” voyages for pelts as might be made “to their infinite gaine” in the Delaware and Hudson, both of which they pointed out were “within our lymits.” The Dutch interlopers on the coasts were almost as challenging to the Virginians, now that they themselves were well rooted, as was the possibility of French competition creeping down the rivers that flowed into Chesapeake Bay from the northern hinterland.
Almost—but not quite as challenging. For one thing, the settlers on the James at this time had neither the ships to trade coastwise nor the trade goods necessary to compete with the Dutch. More importantly, however, right at their back door was the vast tidewater of the bay and, beyond that, great stretches of lake country in the north, all fertile with fur bearers to be had for the taking—provided they got there before the French. This could be accomplished with home-made shallops and less fancy trade goods than were needed to compete with the Dutch.
So it came about that the Virginians left any exploitation of the Delaware and Hudson Rivers to the merchants in England and, individually with their more limited resources, extended their own trading operations in their “Bay of Virginia.”
Scores of immigrants participated in this tidewater fur trade, including a great many who had the advantage of a knowledge of the Algonquin tongue because of having cohabited with the savages. Some of the latter were no more than villainous runaways, cautiously returning from sanctuary with the natives to share the profits of the boom in pelts. Others, such as Robert Poole and Henry Spelman, had been left with the Indians as hostages when they were youths, very much in the same manner as Thomas Savage. Like him, they too had learned the dialects and habits of various tribes, acted as spies for the English colonists and then, later, were useful as interpreters.
Robert Poole traded professionally for the joint-stock, making voyages in the company’s pinnace and rendering his accountings to the colony’s treasurer at Jamestown. But he and Thomas Savage also obtained commissions through members of the council and, renting shallops from more prosperous colonists for voyages to the villages of the Indians, did very well for themselves in this way Probably neither of them traded for pelts as extensively however as did Henry Spelman who eventually managed to get financial backing in England and ships of his own.
Young Spelman was the incorrigible scion of a distinguished English family. According to his own testimony he had been sent out
to Virginia in 1609 at fourteen years of age because of “beinge in displeasure of my fryndes and desirous to see other countries.” Captain John Smith left him with Powhatan’s people, no doubt as the best way to get rid of him. Henry Spelman was not as popular with the natives as Thomas Savage, except perhaps with the Indian maidens. In fact he appears to have escaped with his life only because of the intervention of Pocahontas who, continuing her interest in white men after the affair with Captain John Smith, succeeded eventually to marrying an Englishman named John Rolfe.
Spelman’s entire career in the colony was one of manifold and dangerous deceit. As a youth living with the Indians, he was accused of treacherously laying a death trap for some of his fellow Englishmen at Jamestown. Later, when he was employed as an official interpreter for the colony, his tenure in office was cut short after he was tried by the House of Burgesses and degraded from his rank for inciting Opechancanough, the new Powhatan, against the English governor.
In 1622, when Opechancanough finally loosed his stored-up hatred against the English, killing some three hundred fifty men, women and children along the James River in a surprise massacre, Henry Spelman and a trading partner, Captain Raleigh Croshaw, were bartering in the Potomac. Opechancanough conspired with Chief Japazaws of the Patowomecks to have the traders murdered. But the Patowomeck king double-crossed the Powhatan. Instead of killing the Englishmen, he entered into an alliance with them, permitting them to build a fortified trading post on his lands.
All the evidence would indicate that Japazaws was a very wily politician. He liked English kettles and knives as much as some of his red neighbors disliked him, and he never did get around to knuckling under to Opechancanough. No doubt the fort was a smart move on his part. In any event, because of Japazaws’ need for white friends with guns Captains Spelman and Croshaw escaped death.
Henry Spelman was not so fortunate the following year however. On a trading voyage farther up the Potomac River, at the later site of Washington, D. C., he was killed by the Anacostan Indians after
being captured in an ambush along with Henry Fleet and some other traders and servants. Just a few days before his death Spelman had betrayed a warrior who hazarded warning him of his own intended murder. It appears he even witnessed the agony of the red man’s tortured execution. Then, by some sort of final justice, he died at the hands of the same savages to whom he had given over the kindly warrior.
Aqua vitae may have had something to do with this murderous episode, for, like the French and the Dutch before them and like the English who were to follow in New England, some traders in Virginia taught the savages to drink strong waters as a means of extracting a richer profit during barter. And what a frightful chance they took!
Emotionally volatile and capricious by nature, the red man was even more unpredictable of course when he was under the influence of liquor. He might lose his trading sense to the advantage of the white men who purposely plied him with aqua vitae during the feasting that always preceded barter, but when he realized he had been cheated he was apt to reach for another jug and his tomahawk at the same time. No doubt many a trader suffered a slow and bestial death while looking into the bloodshot eyes of savages galled by stomach fires that he himself had kindled.
Henry Fleet, who had been captured with Spelman, managed the predicament in which he found himself with more skill than his friend, indeed with some foresight. While no one in the colony heard from him in over four years, and all thought he was dead, Fleet not only contrived to escape execution by the savages, but spent his captivity to real advantage. By the time he was finally released he had learned enough about the Algonquin language and had become sufficiently fraternal with his captors to engage in a profitable trade in skins with them. Then, returning to London, where he had excellent family connections, he acquired financial backing from William Cloberry and other well-known merchants there. Afterward, as Captain Henry Fleet, he traded in the Chesapeake for some years in his own ships and later played a significant role in the genesis of the war between the Virginians and Lord Baltimore over the fur trade.
