1step,rule,n_before,n_after,n_removed,rationale21_window,keep months 2001-01..2026-09 of combined monthly series,957,309,648,estimation window defined in TASK.md (combined series spans 1947-01..2026-09)32_missing_level,drop months whose own index level P_t is missing in the source,309,308,1,no imputation allowed; g_t not computable43_missing_lag,drop months whose lagged level P_{t-12} is missing in the source,308,308,0,no imputation allowed; g_t not computable5