THE APPLICATION OF SAMPLE SELECTION MODELS TO OUTCOMES RESEARCH - THECASE OF EVALUATING THE EFFECTS OF ANTIDEPRESSANT THERAPY ON RESOURCE UTILIZATION

Non-randomized studies of treatment effects have come under criticism because of their failure to control for potential biases introduced by unobserved variables correlated with treatment selection and outcomes , This paper describes the basic concepts of sample selection models - a technique used widely in the economics evaluation literature for ne arly two decades - and discusses the potential role of these models in outcomes research. In addition, it presents a case study of the appli cation of the sample selection modelling approach to evaluation of the effects of antidepressant therapies on medical expenditures for physi cian services. This case study presents empirical comparisons of alter native model specifications and discusses practical issues in evaluati on of sample selection models. We demonstrate that, in this particular case, sample selection models yield very different conclusions regard ing treatment effects than traditional ordinary least squares regressi on. (C) 1998 John Wiley & Sons, Ltd.