An introduction to epidemiology.

Epidemiology as defined by Last is "the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the prevention and control of health problems". Traditional epidemiological studies include quantitative and qualitative study designs. Quantitative study designs include observational and interventional methodology. Observational methods describe associations that are already present at population (descriptive) or individual (analytical) level. Although they form the mainstay of epidemiological studies, observational methods are prone to bias and confounding. These can be dealt with by various means involving both the study design and statistical analysis. Interventional methods involve changing variables in one or more groups of people and comparing outcomes between those with the changed and unchanged variable. Interventional studies can more readily account for bias (such as through randomisation) and confounding (such as through controlling) as is seen in randomised, controlled trials. Qualitative studies employ non-numeric methods to obtain "richer" information on how people perceive or experience situations. Much of epidemiology and epidemiological methods have been stable for many years. There are, however, emerging issues in epidemiology, including those of causal inference, counterfactuals and Mendelian randomisation, among others. There are also several modern and emerging uses of traditional epidemiological techniques in the fields of infectious disease, environmental, molecular and genetic epidemiology.