Propensity Score Matching - Statpower

IntroductionModeling the CovariatesSubclassificationMatchingBalancing ScoresThe Propensity ScoreMatching MethodsUsing Propensity Scores – A General StrategyAn ExampleDeciding on Relevant CovariatesDeciding on Relevant Covariates – A CautionA Caution“However, the theoretical literature has focused primarily on thecase where the pool of potential control units is considerablylarger than the set of treated units. Some researchers seem tohave incorrectly generalized this advice to all data sets. If, as isoften the case, the pool of potential control units is not muchlarger than the pool of treated units, then always including allavailable control variables is bad advice. Instead, the familiareconometric rules apply about the trade-off between the bias ofexcluding relevant variables and the inefficiency of includingirrelevant ones: researchers should not include everypretreatment covariate available.”MultilevelPropensity Score Matching