How to assess epidemiological studies.(Review)

Postgrad Med J 2004;80:140-147. doi: 10.1136/pgmj.2003.012633

Assessing the quality of an epidemiological study equates to assessing whether the inferences drawn from it are warranted when account is taken of the methods, the representativeness of the study sample, and the nature of the population from which it is drawn. Bias, confounding, and chance can threaten the quality of an epidemiological study at all its phases. Nevertheless, their presence does not necessarily imply that a study should be disregarded. The reader must first balance any of these threats or missing information with their potential impact on the conclusions of the report.

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Epidemiology underpins good clinical research. It is any research with a defined numerator, which describes, quantifies, and postulates causal mechanisms for health phenomena. (1) Epidemiology gives insight into the natural history and causes of disease and can provide evidence to help prevent occurrence of disease. It promotes effective treatments either to cure or to prolong the lives of those with disease. Epidemiology, also referred to as "population medicine", is used to estimate the individual risk of disease and the chances of avoiding it from group experience averages. Such information is crucial to planning interventions and allocating resources.

The epidemiological approach needs to be applied to clinical research to evaluate both its effectiveness and its importance. Hence clinicians need to gain the skills that will allow them to properly update and re-evaluate their knowledge and thus provide the best evidence based patient care. Epidemiology is an interdisciplinary field that draws its techniques and methodologies from biostatistics, social sciences, and clinical medicine as well as from a vast range of biological sciences such as genetics, toxicology, and pathology (2) and for this reason the interpretation of epidemiological studies is not always easy.

There are several reviews and books available that provide advice on how best to assess epidemiological studies. The favoured outline for these is by listing types of common errors. This review provides an alternative approach that it is hoped will be helpful. After briefly characterising the main threats to the quality of epidemiological studies, a map is provided to assess studies based on their usual format--that is, the design, conduct, and analysis of the results. Readers of epidemiology papers at any level will be assisted in their task by Last's A Dictionary of Epidemiology, an essential guide to all. (1) Assessing the quality of epidemiological studies equates to assessing their validity.

CONCEPT OF VALIDITY AND ITS THREATS