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to more than two time periods. In the DD case, add a full set of time dummies to the equation. This assumes the policy has the same effect in every year; easily relaxed. In a DDD analysis, a full set of dummies is included for each of the two kinds of groups and all time periods, as well as all pairwise interactions. Then, a policy dummy (or sometimes a continuous policy variable) measures the effect of the policy. See Meyer (1995) for applications. With many time periods and groups, a general framework considered by BDM (2004) and Hansen (2007b) is useful. The equation at the individual level is

t g xgtzigtgt vgt uigt, i 1, . . . , Mgt, yigt

where i indexes individual, g indexes group, and t

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