What is a propensity score? Applications and extensions of balancing score methods

Abstract:The foundational propensity score paper by Rosenbaum and Rubin (1983a) laid the foundation for a set of methods widely used in the design of non-experimental studies. This commentary reflects on the theoretical contributions of that paper –especially the idea of the propensity score as a balancing score –as well as on the wide variety of contexts in which the general idea of a balancing score has since been applied. Areas in which the fundamental ideas of a balancing score –which can help equate two groups on the basis of a set of covariates –have been extended include mediation analysis and generalizability. The commentary also touches on common misperceptions regarding propensity scores, and on the key role of the “other” Rosenbaum and Rubin (1983b) paper, which laid out a method for assessing the sensitivity of study results to violation of the key assumption underlying most uses of propensity scores –that of no unmeasured confounding. All together, this body of work has changed how many fields conduct non-experimental studies, and other related types of studies, and with many applications and extensions yet to come.