Attrition bias — MRCPsych Paper A MCQ

Instant feedback + full explanation. One question, done properly.

ModerateEpidemiology and research methodsAttrition biasMRCPsych Paper A

A randomised controlled trial compares online CBT with a control condition. At 12 months, outcome data are unavailable for 30% of the control arm and 8% of the CBT arm. In the control arm, participants without outcome data had substantially more severe symptoms at baseline than those retained. The primary analysis includes only participants with complete follow-up data. Which bias is the principal threat to the estimated treatment effect?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Attrition bias

The correct answer is C, attrition bias. Follow-up differs substantially between trial arms, and missingness is related to baseline symptom severity, so participants included in the complete-case analysis may no longer be comparable. This can systematically distort the estimated treatment effect; the direction of distortion cannot be inferred reliably from attrition rates alone. Observer bias concerns systematic differences in how outcomes are assessed. Publication bias arises when the dissemination of whole studies or results depends on their findings. Ecological fallacy is the incorrect application of group-level associations to individuals. Recall bias results from systematic differences in the accuracy or completeness of remembered information.

Reference: Royal College of Psychiatrists, National Collaborating Centre for Mental Health. Induced Abortion and Mental Health: A Systematic Review, section 2.8 Risk of Bias in Individual Studies, 2011. https://www.rcpsych.ac.uk/docs/default-source/improving-care/nccmh/nccmh---induced-abortion-and-mental-health---systematic-review---2011.pdf?sfvrsn=c7a00ded_3