1step,rule,n_before,n_after,n_removed,rationale21,keep quarters 1959Q4-2019Q4 (GDPC1 quarters as unit),319,241,78,task window; 1959Q4 needed only for lagged levels; post-2019Q4 forbidden32,"drop 1959Q4 (no lagged level, differences undefined)",241,240,1,first-difference construction4