1step,rule,n_before,n_after,n_removed,rationale21,"drop rows with missing/non-finite firm_id, year, x or y",1200,1200,0,FE estimator needs complete cases32,drop exact duplicate rows,1200,1200,0,duplicates would double-weight observations43,drop duplicated firm_id-year keys,1200,1200,0,panel must have one row per firm-year5