Practical propensity score matching: A reply to Smith and Todd

This paper discusses propensity score matching in the context of Smith and Todd's (Does matching overcome Lalonde's critique of nonexperimental estimators, J. Econom., in press) reanalysis of Dehejia and Wahba (J. Am. Statist. Assoc. 97 (1999) 1053; National Bereau of Economics Research working Paper No. 6829, Rev. Econom. Statist., 2002, forthcoming). Propensity score methods require that a separate propensity score specification be estimated for each treatment group-comparison group combination. Furthermore, a researcher should always examine the sensitivity of the estimated treatment effect to small changes in the propensity score specification; this is a useful diagnostic on the quality of the comparison group. When these are borne in mind, propensity score methods are useful in analyzing all of the subsamples of the NSW data considered in Smith and Todd (Does matching overcome Lalonde's critique of nonexperimental estimators, J. Econom., in press).