In a cohort study, confounding by indication is a major concern. A researcher uses propensity score matching to address this. Propensity score is defined as:

• A The probability that a subject develops the outcome given the exposure
• B The conditional probability of receiving the exposure given a set of observed covariates ✓
• C The ratio of outcome probability in exposed versus unexposed after stratification
• D The probability of being a confounder given the observed association

Explanation

A propensity score is the conditional probability of receiving the treatment or exposure given the observed baseline covariates. Matching or weighting on the propensity score creates balance between exposed and unexposed groups on measured confounders, mimicking randomization. It does not eliminate unmeasured confounding. Option A describes the individual risk; option C is a stratified relative risk concept; option D has no standard epidemiological definition.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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