step,rule,n_before,n_after,n_removed,rationale
1,drop quarters before 1959Q4,319,268,51,Not needed; 1959Q4 kept only as lag level for 1960Q1 differences
2,drop quarters after 2019Q4,268,241,27,Information rule: post-2019Q4 quarters outside the question
3,drop 1959Q4 (lag-only row),241,240,1,First differences undefined for the 1959Q4 row in-sample