Rare Outcomes, Common Treatments: Analytic Strategies Using Propensity Scores

When treated patients are compared to controls, dif-fering outcomes may reflect either effects caused by the treatment or differences in prognosis before treatment. Random assignment of patients to treatment or control, as in a randomized, controlled clinical trial (1), ensures that the groups were comparable before treatment and the prognosis in treated and control groups was nearly the same, so that differing outcomes indicate treatment effects. Somewhat more precisely, random assignment ensures that the only differences in prognosis between groups are due to chance, the flip of a coin in assigning treatments. In an ideal randomized trial, if a common statistical test rejects the hypothesis that the difference in outcomes is due to chance, a treatment effect is demonstrated. Notice that randomization does nothing to make patients have individ-ually similar prognoses; rather, it ensures that assignment