Diagnose Medicare Part B premium payer-count shortfall

[daphnehanse11]

Context

The current published Populace US 2024 dataset is low on Medicare Part B premium income:

• gross_medicare_part_b_premium: simulated $127.80B.
• CMS Medicare Trustees target: $139.84B.
• Relative error: -8.6%.

The simulated positive-payer count is about 60.96M people. Given the 2024 standard Part B premium, the average among positive premium payers appears plausible, so this likely looks more like a payer/enrollment-count support issue than a premium-amount formula issue.

This is not covered by active healthcare PRs #165, #166, or #167.

Goal

Diagnose and fix the Medicare Part B premium target miss by separating payer-count support from premium amount per payer.

Suggested implementation

• Add a Medicare Part B premium diagnostic to the US fiscal refresh release artifacts:
  • positive gross_medicare_part_b_premium payer count,
  • aggregate premium amount,
  • average premium per positive payer,
  • target-implied payer count using the standard premium when appropriate.
• Inspect whether Populace is missing a Medicare Part B enrollment/payer input column, or whether PE-US derives payer status from a broader Medicare eligibility/enrollment variable.
• If the issue is source support, add or materialize the appropriate input; if it is formula-owned, document the boundary and open/link a PE-US issue.

Acceptance criteria

• Release diagnostics identify whether the miss is payer count, premium amount per payer, or both.
• A focused test covers the diagnostic payload.
• The current $127.80B vs $139.84B gap has an actionable owner: Populace source data if payer support is missing, PE-US if the model formula is responsible.
• If Populace owns the fix, a follow-up implementation improves the Medicare Part B premium target without changing unrelated healthcare targets.