Tous les séminaires
Jeudi 14 Octobre 2010
Sylvain CHABE-FERRET (CEMAGREF, UMR METAFORT, Aubière) présentera une communication :
"To Control or Not to Control ? Bias of Simple Matching vs Difference-In-Difference Matching in a Dynamic Framework"
- de 14 h à 15 h 30 en Salle S016 à l'INSEE-CREST, 15 Boulevard Gabriel Péri, 92245 MALAKOFF (Métro : Malakoff/Plateau de Vanves (Immeuble "Malakoff 2)).
Abstract : In this paper, I examine whether the claim “the more control variables, the better” holds when matching in a dynamic context. I exhibit three situations where it is better to difference with respect to past outcomes rather than to control for them: DID-matching is unbiased whereas matching on past outcomes is biased. I also study the special case of evaluating a job training program, borrowing a credible selection rule from Heckman, LaLonde, and Smith (1999) and relying on the parameters of the wage process estimated by MaCurdy (1982). I derive closed forms for the bias terms of the two estimators when the error terms are normally distributed. I show that DID matching is unbiased when applied symmetrically around the period of enrollment, as implemented by Heckman, Ichimura, Smith, and Todd (1998). DID-matching is more robust than matching to misspecification both of the amount of information individuals have when deciding to enter the program and of the control period. I finally point to previously unnoticed experimental results that confirm the claim that DID-matching is less biased than matching.