Principles for modeling propensity scores in medical research: a systematic literature review.

PURPOSE: To document which established criteria for logistic regression modeling researchers consider when using propensity scores in observational studies.
METHODS: We performed a systematic review searching Medline and Science Citation to identify observational studies published in 2001 that addressed clinical questions using propensity score methods to adjust for treatment assignment. We abstracted aspects of propensity score model development (e.g. variable selection criteria, continuous variables included in correct functional form, interaction inclusion criteria), model discrimination and goodness of fit for 47 studies meeting inclusion criteria.
RESULTS: We found few studies reporting on the propensity score model development or evaluation of model fit.
CONCLUSIONS: Reporting of aspects related to propensity score model development is limited and raises questions about the value of these principles in developing propensity scores from which unbiased treatment effects are estimated. Copyright 2004 John Wiley & Sons, Ltd.