ATT estimates are obtained after propensity score matching. Columns (1)–(3) report ATT estimates, robust standard errors, and p-values computed using standard errors clustered at the matched-set (subclass) level, allowing for arbitrary correlation within each matching set. Columns (4)–(6) report ATT estimates, standard errors, and p-values obtained from a matched-set bootstrap, where resampling is performed at the level of matched sets rather than individual observations. This procedure preserves the dependence structure induced by the matching process