The propensity score is defined to be a subject's probability of treatment selection, conditional on observed baseline covariates.  Conditional on the propensity score, treated and untreated subjects have similar distributions of observed baseline covariates.

In the medical literature, there are three commonly employed propensity-score methods: stratification (subclassification) on the propensity score, matching on the propensity score, and covariate adjustment using the propensity score.  Methods have been developed to assess the adequacy of the propensity score model in the context of stratification on the propensity score and propensity-score matching.  However, no comparable methods have been developed for covariate adjustment using the propensity score.  Inferences about treatment effect made using propensity-score methods are only valid if, conditional on the propensity score, treated and untreated subjects have similar distributions of baseline covariates.

The authors develop both quantitative and qualitative methods to assess the balance in baseline covariates between treated and untreated subjects.  The quantitative method employs the weighted conditional standardized difference.  This is the conditional difference in the mean of a covariate between treated and untreated subjects, in units of the pooled standard deviation, integrated over the distribution of the propensity score.  The qualitative method employs quantile regression models to determine whether, conditional on the propensity score, treated and untreated subjects have similar distributions of continuous covariates.

The authors illustrate their methods using a large dataset of patients discharged from hospital with a diagnosis of a heart attack (acute myocardial infarction).  The exposure was receipt of a prescription for a beta-blocker at hospital discharge.