Case-control studies involve ‘cases' being compared to ‘controls' with respect to ‘exposures', possible aetiological (or associated) factors. Associations between a disease and an exposure can be explained by chance, reverse causality, confounding and biases or, lastly, by causality. However, confounders as well as information and selection biases can be adjusted for at the design (or analysis) stage of the study. The strength of an association can be measured by means of relative risk, calculated indirectly using the odds ratio. Well conducted control studies should produce accurate estimates of relative risks in many psychiatric investigations.