skip to main content

Confounding in observational study — USMLE Step 3 MCQ

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

ModeratePreventive Medicine, Epidemiology, BiostatisticsConfounding in observational studyUSMLE Step 3

A cohort study evaluates whether a new discharge checklist reduces 30-day readmission after heart failure hospitalization. The intervention hospital has lower readmission than the control hospital, but patients there are younger and have fewer comorbidities. Investigators adjust for age, renal disease, and ejection fraction. Which of the following is the most likely explanation for this patient's findings?

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

Reveal the answer and explanation

Correct answer: B — Confounding by baseline patient differences

Unequal baseline risk factors that are associated with both the intervention setting and outcome can confound observational comparisons. Statistical adjustment attempts to reduce this bias.

Reference: USMLE Step 3 biostatistics content outline