step,rule,n_before,n_after,n_removed,rationale
1,calendar months 2001-01..2026-09 (window),309,309,0,starting set: every month of the estimation window
2,drop months with no index level P_t in latest covering vintage,309,308,1,no interpolation/imputation allowed (e.g. 2025-10 empty in 2026-10-28 vintage)
3,drop months with no lagged level P_{t-12},308,308,0,g_t requires both P_t and P_{t-12}