While conducting empirical work, researchers sometimes observe changes in outcomes before adoption of a new treatment program. The conventional diagnosis is that treatment is endogenous. Observing changes in outcomes prior to treatment is also consistent, however, with anticipation effects. This paper provides a framework for comparing the different methods for estimating anticipation effects and proposes a new set of instrumental variables that can address the problem that subjects\u27 expectations are unobservable. The paper uses this framework to analyze the effect of tort reform on physician supply and finds that accounting for anticipation effects doubles the estimated effect of tort reform.
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