coefficient on wit is t ai. Can eliminate ai (and ci0. Or, with t , can “estimate” the i and then use
̂ N−1 ∑̂i.
See Wooldridge (2002, Section 11.2) for standard error, or bootstrapping. 5. Semiparametric and Nonparametric Approaches ∙ Return to the setting with two groups and two time periods. Athey and Imbens (2006) generalize the standard DD model in several ways. Let the two time periods be t 0 and 1 and label the two groups g 0 and 1. Let Yi0 be the counterfactual outcome in the absense of intervention and Yi1 the counterfactual outcome with intervention. AI