ESD - Case Control Studies Lesson

Case Control Studies

A case-control study is an investigation that compares a group of people with a disease to a group of people without the disease. The frequency with which the exposure occurred is compared between the cases and the controls. The purpose of a case-control study is to determine if there are one or more factors associated with the disease under study. Information about prior exposure may be obtained by a variety of methods, including self-administered questionnaires, interviews, and medical examinations. Because in case-control studies, information about exposure is generally collected after the disease has already occurred, these studies are sometimes called retrospective studies.

For example, a case-control study of obesity in high school students may identify a group of students who suffer from obesity and compare them to a control group of students who are not obese in regard to factors such as exercise habits, eating habits, and family history of obesity. The intent would be to determine if exposure to one of these factors is different in the cases and controls. If so, that factor is said to be associated with obesity and may influence the occurrence of the disease.

Case-control studies have several advantages. They can be used to study rare diseases, are relatively inexpensive, and are particularly useful for studying diseases with long incubation periods. The chronological order of the exposure and disease is easy to determine in cohort studies, but can be uncertain from the results of a case-control study because it may not be possible to know if the exposure occurred before the disease. In practice, case-control studies are often affected by selection bias. Selection bias may occur if the control group does not come from the same population as the cases. Information bias is another common problem of case-control studies. A type of information bias often found in case-control studies is recall bias. Recall bias may arise when there is a difference in how individuals with a disease remember past exposures versus how controls remember past exposures. This often happens because sick individuals try to figure out what caused their disease. In contrast, individuals without disease have no special reason to remember past exposures.  

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