Measures of disease frequency and rate in epidemiology studies
Measures ď ľ
The data obtained from an epidemiology study can be divided into:
Qualitative-catagorical
Nominal The data can only be classified without being ranked. Examples are, "male or female", "smoker or non-smoker", "drug or placebo" and the general "yes or no" answers. Ordinal The data can be ranked, but with no equal space between the ranks. Used to describe magnitude of measured item. Examples are, socioeconomical status: "poor, middle class, rich" or the degree of skin burn: "first degree, second degree, third degree".
qualitative-continuous
Interval Interval scales are numeric scales in which the order to the exact is known. the data have equal space between unit and have no absolute zero. Examples are, temperature or date of birth. Ratio An interval variable with a true zero point. Example are height and weight.
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The variables collected from any study will be summarized into frequency measures such as ratios, proportions, and rates.
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Incidence, prevalence, and mortality (and morbidity) rates are three frequency measures that are used to characterize the occurrence of health events in a population.
Incidence ď ľ
Incidence in epidemiology is a measure of the probability of occurrence for a given medical condition in a population within a specified period of time.
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Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.
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Cumulative incidence rate (number of new cases in a specified time period divided by number of population that is at risk of the disease)
Cumulative incidence rate
Incidence Rate ď ľ
direct estimate of probability or risk of developing disease during a specified time period
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Incidence rate or density (number of new cases in a specified time period divided by the total number of person-time when at risk)
Prevalence ď ľ
Prevalence in epidemiology is the proportion of a population found to have a condition (typically a disease or a risk factor such as HIV-positive or smoking).
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It is arrived at by comparing the number of people found to have the condition with the total number of people studied, and is usually expressed as a fraction, as a percentage or as the number of cases per 10,000 or 100,000 people. It is an indication of how the number of cases relates to the population.
Prevalence rate ď ľ
the total number of cases during a specified time period divided by the population count.
Mortality and Morbidity ď ľ
Morbidity refers to the unhealthy state of an individual, while mortality refers to the state of being mortal. Both concepts can be applied at the individual level or across a population.
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For example, a morbidity rate looks at the incidence of a disease across a population and/or geographic location during a single year. Can be measured by incidence rate, prevalence rate and disability number.
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Mortality rate is the rate of death in a population. The two are often used together to calculate the prevalence of a disease (eg., measles) and how likely that disease is to be deadly, particularly for certain demographics.
Mortality rate
In terms of a single year and a population of 1000 or RATIO! Unit of time must be specified. Can be made explicit for characteristics i.e., age, gender, marital status, ethnicity and specific causes
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