1step,rule,n_before,n_after,n_removed,rationale21,keep quarters 1959Q4-2019Q4 (levels window; information rule),319,241,78,task fixes regression window 1960Q1-2019Q4; 1959Q4 levels needed only for first differences32,drop 1959Q4 (no prior level for differencing; levels-only quarter),241,240,1,1959Q4 is used only as the base level for 1960Q1 differences4