Research: Hierarchy of Evidence
It is heartening to witness a display of papers written by medical students. It may be salutary for the younger colleagues to read a reminder as to how the hierarchy of evidence is viewed by clinicians and researchers alike and for this reason, the editorial will briefly outline this pyramid. 1.1. Observational studies Observational studies describe a naturally occurring variation within a given population. Types of observational studies include: • Case report or case series • Ecological studies • Cross-sectional studies and surveys • Case-control studies • Cohort studies 2.1.1 Case series A case report or series is a descriptive study that does not involve any hypothesis testing. Instead the study observes subjects with a known exposure (such as a type of medication) and records outcome data - selection bias is inherent. 2.1.2 Ecological studies Ecological studies describe the associations between an exposure and an outcome at a group level such as by time trends or by geographical location. However, such studies cannot confirm associations. 2.1.3 Cross-sectional studies Cross-sectional studies record data at one point in time, a snapshot. This type of study is easy to conduct and relatively cheap. Conducting regular cross-sectional randomised population studies may provide useful health information on exposures and outcomes. However, temporal relationships between the exposure and the outcome cannot be confirmed. 2.1.4 Case-control studies Case-control studies retrospectively explore an exposure between the individuals with a disease (case) and those without the disease (control). This is relatively quick and cheap but susceptible to selection and recall bias, and temporal relationships between exposure and the outcome cannot be assessed. 2.1.5 Cohort studies Cohort studies (or longitudinal studies) follow a group of people (healthy or otherwise) over a period of time, in order to measure the incidence of a particular disease (if healthy) or to
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