Epidemiology is the study of the **distribution and determinants of health-related states and events in specified populations**.

Last's Dictionary of Epidemiology (2000)

Epidemiologists use a rich language to describe how they apply statistical methods to the study of populations in order to work out, for example, the causes of diseases.

If a population is exposed to some factor, called the exposure, the Epidemiologists usually study the relationship between the exposure and relevant heath outcomes, for example cigarette smoking and lung cancer.

A very important question that Epidemiologists must ask themselves when thinking about a numerical association between some exposure(s) and outcome(s) is "how might I be wrong". The answer is by:

You should understand the basic concepts of causality, chance, bias and confounding in order to start to work with epidemiological problems. You should also understand the basic principles of study design, for example prospective vs. retrospective studies.

There are several introductory text books, either under a cover title of Epidemiology or Statistics (e.g. Bland 2000) and web sites.

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