"The central role of the propensity score in observational studies for causal effects" by Rosenbaum and Rubin, 1983 (link)
"Causal inference with general treatment regimes" by Imai and van Dyk, 2004 (link)
"Generalized propensity score for estimating the average treatment effect of multiple treatments" by Feng et al, 2012 (link)
"Bias associated with using the estimated propensity score as a regression covariate" by Lu and Hade, 2014 (link)
"The role of the propensity score in estimating dose-response functions" by Imbens, 2001 (link)
"Program heterogeneity and propensity score matching: An application to the evaluation of active labor market policies" by Lechner, 2002 (link)
"A tutorial on propensity score estimation for multiple treatments using generalized boosted models" by McCaffrey et al, 2014 (link)
"One‐to‐many propensity score matching in cohort studies" by Rassen et al, 2012 (link)
"Estimation of causal effects with multiple treatments: a review and new ideas" by Lopez and Gutman (under review), 2015 (link)
"Matching with doses in an observational study of a media campaign against drug abuse" by Lu et al, 2001 (link)
"Invited commentary: propensity scores" by Joffe and Rosenbaum, 1999 (link)
"Using propensity score subclassification for multiple treatment doses to evaluate a national antidrug media campaign" by Zanutto et al, 2005 (link)
"Estimating the average treatment effects of nutritional label use using subclassification with regression adjustment" by Lopez and Gutman, 2013 (link)