step,rule,n_before,n_after,n_removed,rationale
1,drop rows with missing firm_id/year/x/y,1200,1200,0,FE estimator needs complete cases
2,drop all rows of duplicated firm_id-year keys,1200,1200,0,panel must have one row per firm-year
3,drop singleton firms,1200,1200,0,singletons carry no within-firm information