Most individual-level studies use case-control or response-based study designs to study rare events, such as completed suicide. However, the strengths and weaknesses of this study design are not well understood by investigators outside the public health community, and in order to clarify the controversy surrounding some of these studies, it may be helpful to describe the most important features of the case-control study design. Studies of the rates and determinants of illness or behaviors can be classified as retrospective or prospective. Prospective studies usually select people on the basis of exposure and determine how many persons with the exposure, compared with persons without exposure, develop a certain outcome. In contrast, retrospective studies usually start by choosing persons according to whether an illness or behavior has already developed and seek to find the phenomena that might be associated with the development of the outcome. Intuitively, it makes sense that if one is studying a rare outcome, then a prospective design is inefficient because it may take a very large sample or a very long time to accumulate enough occurrences. In this case, the case-control sampling design is beneficial because it oversamples the behavior or outcome of interest.
To investigate suicide, for example, a case-control study might select as cases those persons who have committed suicide, and then randomly select as controls a certain predetermined number of subjects from the same population who did not commit suicide. The study design would seek to establish an association between the outcome (suicide) and an exposure (such as firearms or depression) by noting the proportions of cases and controls that have been exposed to the possible risk factor.
There are a number of important advantages to the case-control method that explain its common use in epidemiology. Because the outcomes have already happened, case-control studies require no costly follow-up waiting for the outcome to develop. Because case-control studies oversample the outcome of interest, they also require smaller samples sizes than prospective studies of comparable power; for this reason, the case-control sampling scheme is often the only feasible way to collect the information of interest. For example, although suicide is the most common cause of firearm-related deaths in the United States, the overall suicide rate is approximately 11 suicides per 100,000 persons per year. Very few prospectively collected data sets would be large enough to draw precise inferences about completed suicide.
Feasible and efficient as the case-control design may seem, it also suffers from important limitations arising from the nonrandom selection of cases or controls and from misclassification of the outcome or exposure.