Medicare: add a Part B enrollment input and impute SSDI months so Part B rules reach the right people #10457

Problem

Two gaps in the Medicare inputs bias any rule that depends on Part B enrollment. California's Medi-Cal aged and disabled FPL program is one such rule. It disregards the Part B premium from countable income (ACWDL 20-18 and 20-24), and PR 10364 encodes that disregard.

1. No Part B enrollment input. medicare_enrolled covers Part A and/or Part B (adds = ["takes_up_medicare_if_eligible"]). The Part B chain uses it as the Part B gate: medicare_part_b_premium, msp_part_b_premium_coverage and medical_expense_health_insurance_premiums all do, and so does the California pathway in PR 10364. People enrolled only in Part A therefore get Part B premiums and the California Part B disregard. KFF and CMS data for 2024 show about 10% of California Medicare beneficiaries with Part A only.
2. No SSDI duration in the microdata. is_medicare_eligible reaches people under 65 only through months_receiving_social_security_disability >= 24. No microdata build sets that input, so in simulation only people 65 or older reach Medicare. Issue 9396 covers documenting this input and adding the ESRD and ALS routes. This issue asks for the data side.

Size of the effect (California pathway, 2026, default dataset)

The round 2 review of PR 10364 estimated the effect:

• About 14,000 people are overstated as eligible because Part A-only enrollees get the Part B disregard. That is about 0.65% of California pathway eligibles, and about 7,000 of them have no other Medicaid category. The estimate rests on 11 records, so it could be off by a factor of about 2.
• About 518,500 California disabled beneficiaries under 65 never reach is_medicare_eligible, so they get no Part B disregard. At the same rate this understates about 12,000 people.
• The net effect is close to zero, and its sign is uncertain.

Proposed work

• Add a Part B enrollment input, or split Medicare take-up into Part A and Part B. Then gate the Part B chain on it (gross_medicare_part_b_premium, medicare_part_b_premium, msp_part_b_premium_coverage, medical_expense_health_insurance_premiums, and the California pathway's base_part_b_premium term).
• Impute months_receiving_social_security_disability (or a 24-month flag) in the microdata build, so that SSDI recipients under 65 reach is_medicare_eligible.
• Add YAML cases: a Part A-only enrollee with no Part B premium, and an SSDI recipient under 65 with 24 or more months who is Medicare eligible.

Related

• Update Medicaid aged, blind and disabled pathway disregards and asset limits (CA, CT, MN, MO, AR, OK, SC) #10364: California pathway Part B disregard. The code there carries a NOTE about the medicare_enrolled stand-in.
• Issue 9396: Medicare eligibility routes and the SSDI-months input.
• Issue 7967: Medicare Part B premiums in baseline SPM MOOP.