What the plan design looked like for the SAME Medicare Advantage members in year Y and in year Y+1, member-weighted from the CMS Landscape files: total, Part C and Part D premiums and the $0 shares, the premium after full Extra Help, in-network MOOP and the share at or under $5,000, the Part D deductible and its $0 and standard shares, the tier-skips-deductible share, MA-PD and enhanced shares, the D-SNP $0 cost sharing, FIDE and HIDE shares, and the overall star. Both sides of a change are the SAME members: y = the Y design of the members whose plan continues in their county (same_store, Day-One) or is routed through the CMS crosswalk where that crosswalk is held (routed: continues + moved); y1 = those members' Y+1 design; change = y1 minus y. all_members_y = the Y design on every September-Y member (members_y), context only, never one side of a change. The members whose plan departs and is not yet crosswalked are in all_members_y and members_y but NOT in y, y1 or members_y1, and the row says how many.

ARGUMENTS: transition_year (Y, default the newest held), group_by (national | product_type | parent | state | parent_by_product | state_by_product | county; county needs state and returns one row per county of that state, each on its own members), product_type / state / parent (filters), county (filter, the county name as CMS spells it, e.g. 'Maricopa'; with product_type it cuts one county by product), metrics ('key' = the twelve headline metrics, 'all' = every metric, or a comma-separated list of metric names), weight (same_store | routed; any other value is refused by name, and routed is refused where the crosswalk is not held), include_cost_mmp (default false), limit (default 50, max 500, largest members first).

RETURNS value.rows (one per group: members_y (every September-Y member), members_y1 (the same members on both sides of the change) and, per metric, {y, y1, change, all_members_y, unit}), value.summary (the national row for the same scope, the editions, the weight used) and value.metrics (the metric labels and units). Dollars are the filed monthly premium / annual MOOP or deductible in nominal dollars; pct metrics are shares of members in the metric's own domain (Part D metrics over MA-PD members, D-SNP metrics over D-SNP members).

RULES THE TOOL ENFORCES: member base H/R contracts, plans 001-799, matched to the Y file by plan and county; PDP never; industry excludes Cost and MMP unless asked; D-SNP is its own product type; a change is over the same members, never a mix of two populations. Suppressed cells count as zero, so every weight is a floor.

SOURCE: Vlada analysis of the CMS MA/Part D Landscape files, CMS monthly enrollment by plan and county, and the CMS plan crosswalk where held.