Number needed to treat — MRCPsych Paper A MCQ

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ModerateEpidemiology and research methodsNumber needed to treatMRCPsych Paper A

In a six-month placebo-controlled trial, 14% of participants receiving placebo relapse, compared with 0% receiving the active medicine. Which statement about the treatment effect is correct?

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Correct answer: C — The number needed to treat is 8

The correct answer is C. The absolute risk reduction is 14 percentage points: 0.14 minus 0. Therefore, NNT = 1/0.14 = 7.14, which is rounded up to 8 patients because treating only seven patients would not, on average, prevent one complete additional relapse. Option A confuses the 86% proportion remaining relapse-free on placebo with the absolute risk reduction. The treated-to-placebo relative risk is 0/0.14 = 0, not 7. A hazard ratio cannot be calculated from the two fixed-time cumulative risks alone, so option D is unsupported. Specificity is a diagnostic-test measure and is not applicable to this treatment trial.

Reference: Batelaan NM et al. Risk of relapse after antidepressant discontinuation in anxiety disorders, obsessive-compulsive disorder, and post-traumatic stress disorder: systematic review and meta-analysis of relapse prevention trials. Methods: Number needed to treat. BMJ 2017;358:j3927. https://www.bmj.com/content/317/7168/1309