Hello, I am looking to generate propensity score in R using ps() command. My question comes from how to include categorical variables in the equation to generate propensity score. How should I include reference category in the estimation? Should I drop the reference group manually before running the estimation or R would automatically omit it as if I include it in the estimation? I look forward to your response! Qian Zhang Department of Sociology University of South Carolina [[alternative HTML version deleted]]