Case-control and cross-sectional studies are two important observational study designs used in epidemiology and public health to investigate the distribution, determinants, and burden of diseases in populations. Both designs are particularly useful when experimental studies are difficult, unethical, or impractical. However, they differ in their method of selecting participants, the timing of exposure and disease measurement, and the types of epidemiological questions they can answer. A case-control study begins with the identification of individuals who already have a particular disease or health outcome. These individuals are called cases. A comparison group consisting of individuals who do not have the disease is selected and called controls. The investigator then looks backward to determine whether the participants had been exposed to a suspected risk factor. Thus, case-control studies generally proceed from outcome to exposure. They are especially useful for studying rare diseases or diseases with long latency periods. For example, if an epidemiologist wants to investigate whether exposure to a particular environmental chemical is associated with a rare cancer, people diagnosed with that cancer can be selected as cases. Individuals without the cancer but similar in important characteristics can be selected as controls. Their previous exposure to the chemical can then be compared. If exposure is substantially more common among cases than controls, this may provide evidence of an association between the exposure and the disease. The major measure of association in a case-control study is the odds ratio. It estimates the strength of association between an exposure and an outcome. An odds ratio greater than one may suggest that exposure is associated with increased odds of disease, whereas an odds ratio below one may suggest a possible protective association. Case-control studies are relatively quick and inexpensive compared with many cohort studies. They also allow researchers to investigate several possible exposures for a single disease. However, case-control studies have limitations. Selection of appropriate controls can be difficult because controls should represent the population from which the cases arose. Recall bias can occur when people with a