Invited commentary: Propensity scores

The propensity score is the conditional probability of exposure to a treatm ent given observed covariates. In a cohort study, matching or stratifying t reated and control subjects on a single variable, the propensity score, ten ds to balance all of the observed covariates; however, unlike random assign ment of treatments, the propensity score may not also balance unobserved co variates. The authors review the uses and limitations of propensity scores and provide a brief outline of associated statistical theory. They also pre sent a new result of using propensity scores in case-cohort studies.