1step,rule,n_before,n_after,n_removed,rationale21,keep quarters 1959Q4-2019Q4 (levels window),319,241,78,task window 1960Q1-2019Q4 plus 1959Q4 levels for first differences; window not extended32,drop 1959Q4 (no first difference; levels only),241,240,1,1959Q4 used only as lag level for 1960Q1 differences4