Experimental researchers routinely use regression in order to control for pre-treatment covariates. This practice has become controversial in the wake of recent demonstrations showing that this type of regression is prone to bias in small samples. Bias may even remain when units are sampled from...

The appeal of regression-discontinuity (RD) design arises from its interpretation as a "local natural experiment." However, even many flexible forms of parametric estimation do not yield simple and transparent analysis in the spirit of this interpretation. While influential work in econometrics...

This paper discusses whether differences in the data structure of observational and experimental studies should lead to different strategies for variable selection. On the one hand, it is argued that outliers in the predictor variables have to be treated differently in the two kinds of studies....