1step,rule,n_before,n_after,n_removed,rationale21,drop rows with missing values in any column,1200,1200,0,complete cases required (none expected)32,drop duplicated firm_id,1200,1200,0,one row per firm43,drop training_hours < 0,1200,1188,12,negative hours of training are logically impossible; treated as invalid treatment records5