1step,rule,n_before,n_after,n_removed,rationale21,All calendar months in window 2001-01..2026-09,309,309,0,Starting set: one row per month of the estimation window32,Drop months whose g_t cannot be computed (P_t or P_{t-12} absent in all vintages),309,308,1,No interpolation/imputation allowed; missing: 2025-104