1step,rule,n_before,n_after,n_removed,rationale21,keep quarters 1959Q4-2019Q4 (GDPC1 quarters with complete 3-month UNRATE average),319,241,78,window fixed by TASK.md; 1959Q4 kept only as difference base32,drop 1959Q4 (no lagged level for first difference),241,240,1,first differences need t-1 levels; regression sample 1960Q1-2019Q44