Case-control study odds ratio — MRCPsych Paper A MCQ

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EasyEpidemiology and research methodsCase-control study odds ratioMRCPsych Paper A

A study recruits people with first-episode psychosis as cases and people without psychosis as matched controls. Previous cannabis exposure is ascertained in both groups. Which statistical measure most appropriately quantifies the association between cannabis exposure and psychosis case status?

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Correct answer: B — Odds ratio

The correct answer is **A, odds ratio**. This is a case-control study because participants are selected according to outcome status—first-episode psychosis or no psychosis—and their previous cannabis exposure is then compared. The odds ratio quantifies the exposure odds among cases relative to the exposure odds among controls. Matching should be accounted for in the analysis, commonly using conditional logistic regression, but the resulting measure remains an odds ratio. Relative risk requires incidence or risk denominators in exposed and unexposed groups, which are not directly available from conventional case-control sampling. Cronbach alpha assesses the internal consistency of a multi-item scale. A mean paired difference applies to paired continuous measurements, while median survival time requires time-to-event data.

Reference: Grant RL. Converting an odds ratio to a range of plausible relative risks for better communication of research findings. BMJ. 2014;348:f7450. https://www.bmj.com/content/348/bmj.f7450.short