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P value interpretation — MRCPsych Paper A MCQ

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HardEpidemiology and research methodsP value interpretationMRCPsych Paper A

A randomised trial compares relapse rates between two treatments using a prespecified two-sided test. The null hypothesis is that the treatments have equal relapse rates. The analysis gives p = 0.03, with a significance threshold of 0.05. Which statement most accurately interprets this p value?

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Reveal the answer and explanation

Correct answer: E — If the null hypothesis and test assumptions hold, a result this extreme or more extreme has a 3% probability.

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

D is correct. A p value is conditional on the null hypothesis and the assumptions of the statistical test: if these hold, p = 0.03 means there is a 3% probability of obtaining a test result at least as extreme as that observed. Because 0.03 is below the prespecified 0.05 threshold, the result is statistically significant. It does not quantify the probability that the null hypothesis is true or false, so C is incorrect. Clinical importance requires an effect estimate and confidence interval, excluding A. Power concerns the probability of detecting a specified effect and cannot be inferred as 97% from this p value, excluding B. Statistical significance also provides no assurance against bias, confounding or methodological error, excluding E.

Reference: Biau DJ, Jolles BM, Porcher R. P value and the theory of hypothesis testing: an explanation for new researchers. Clinical Orthopaedics and Related Research. 2010;468(3):885–892. https://pubmed.ncbi.nlm.nih.gov/19921345/