1. • A case-control study is a retrospective observational study and is an alternative to a prospective observational study. Cases are identified in an underlying population and a comparable control group is sampled.

2. • In the standard design exposure information is obtained retrospectively, though this is not necessarily the case if the case-control sample is nested within a prospective cohort.

3. • Prospective studies are not cost effective for rare outcomes. By contrast, in a case-control study the ratio of cases and controls is higher than in the underlying population in order to make more efficient use of resources.

4. • There are two main types of case-control design; matched and unmatched.

5. • The odds ratio is the most commonly used measure of association between exposure and outcome in a case-control study.

6. • Important extensions to the standard case-control design include the explicit incorporation of time into the choice of controls and into the analysis.

Defining a case-control study

Consider a population of interest, for example the general population of the UK, perhaps restricted by gender or age group. We may call a representation of the process by which exposures X and outcomes Y occur in the presence of intrinsic features W the population model. As noted in the Preamble, such a system may be investigated prospectively or retrospectively; see Figure 1.1. In a prospective or cohort study a suitable sample of individuals is chosen to represent the population of interest, values of (W, X) are determined and the individuals are followed through time until the outcome Y can be observed.