In pharmacoepidemiology, cohort studies and case-control studies have been commonly used as research methods to examine causal relationship between exposures to medicines and occurrences of advance events. For both study designs, we could assume a common population at risk, in which cases are developed. A cohort study defines a research cohort within the population at risk and tries to investigate the research cohort directly, and a case-control study tries to investigate the research cohort partly by the control sampling. Assuming an underling research cohort, it becomes possible to understand cohort studies and case-control studies within an unified framework. We revisited several sampling methods to select controls in case-control studies and effect measures implied by the sampling methods. (Jpn J Pharmacoepidemiol 2013; 18(2): 95-111)
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