Brookhart MA, Schneeweiss S, Rothman KJ, Glynn RJ, Avorn J, Sturmer T. Variable selection for propensity score models. Am J Epidemiol. 2006 Jun 1;163(12):1149-56.
Rothman KJ. A sobering start for the cluster busters' conference. Am J Epidemiol. 1990 Jul;132(Suppl 1):6-13.