Assess ability to apply difference-in-differences (DiD) to estimate the causal effect of regulatory changes on firm-level productivity: specify treatment and control groups, write the DiD regression with fixed effects and interaction terms, justify the parallel-trends assumption and propose empirical checks (event-study, pre-trend tests), handle staggered adoption and clustered SEs, interpret ATT from output, calculate a simple ATT from summary statistics, and recommend robustness checks against selection, anticipation, and spillovers.