Cohort study relative risk — MRCPsych Paper A MCQ

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EasyEpidemiology and research methodsCohort study relative riskMRCPsych Paper A

A five-year cohort study follows patients with depression who are initially free of dementia. The researchers calculate the five-year cumulative incidence of dementia separately in patients with and without chronic benzodiazepine exposure. Which effect measure is obtained by dividing the cumulative incidence in the exposed group by that in the unexposed group?

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Correct answer: D — Risk ratio

Explanation lettering: E = shown as A · A = shown as B · D = shown as C · C = shown as D · B = shown as E

C is correct. The risk ratio, also called relative risk, is the cumulative incidence of the outcome in the exposed group divided by the cumulative incidence in the unexposed group. A cohort design permits direct estimation of incidence because participants are followed over time. The risk difference instead subtracts the two cumulative incidences. A prevalence ratio compares existing cases at a specified time rather than new cases arising during follow-up. An incidence rate ratio compares rates based on person-time, which the stem does not describe. An odds ratio compares outcome odds and is particularly associated with case-control studies, although it may also be calculated in other designs.

Reference: Keogh RH and Cox DR. Design choices for observational studies of the effect of exposure on disease incidence. BMJ Open. 2019;9:e031031. https://bmjopen.bmj.com/content/9/12/e031031