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Submitted by Dimitriy Masterov on

This is an excellent summary of this paper. I have a follow-up question about DDD. In Jeff Wooldridge's Econometric Analysis (2nd edition), he gives an example of a difference-in-difference-in-differences (DDD) estimator on page 151 for the two period case where state B implements a health care policy change aimed at the elderly. If I want to extrapolate to what this would do to the elderly in other states, how should I cluster?