Subtract the expected Part D premium from medicare_cost

changelog.d/medicare-cost-part-d-premiums.fixed.md

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+Subtract Medicare Part D premiums from `medicare_cost`, whose CMS spending per enrollee includes Part D: the average Part D premium per Medicare enrollee from the 2026 Medicare Trustees Report, plus the person's own Part D income-related monthly adjustment amount.

policyengine_us/parameters/calibration/gov/hhs/medicare/part_d/enrollment_share.yaml

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+description: The Medicare Trustees' reported and projected enrollment gives this share of Medicare enrollees, people with Part A or Part B coverage, who are enrolled in Part D.
+metadata:
+  unit: /1
+  period: year
+  label: Medicare Part D enrollment share, including retiree drug subsidy beneficiaries
+  reference:
+  # Table V.B3, "Medicare Enrollment": Part D enrollment over total
+  # enrollment, which footnote 2 defines as the "Number of beneficiaries with
+  # HI and/or SMI coverage". Part D enrollment includes retiree drug subsidy
+  # beneficiaries (it equals the total of Table IV.B7, file page 153).
+  # Annual through 2035, then every fifth year through 2100; each value holds
+  # until the next. 2026 Medicare Trustees Report, transmitted 2026-06-09,
+  # retrieved 2026-10-11, sha256
+  # ffa56b9137006872300b0346149eae1613d09a172b6ba118aad48e66dfc48fa8.
+  - title: "2026 Annual Report of the Boards of Trustees of the Federal Hospital Insurance and Federal Supplementary Medical Insurance Trust Funds, Tables IV.B7 and V.B3 #page 153,196"
+    href: https://www.cms.gov/oact/tr/2026#page=153
+values:
+  # Historical.
+  2015-01-01: 0.7517
+  2016-01-01: 0.7569
+  2017-01-01: 0.7579
+  2018-01-01: 0.763
+  2019-01-01: 0.7667
+  2020-01-01: 0.7743
+  2021-01-01: 0.7811
+  2022-01-01: 0.7885
+  2023-01-01: 0.7941
+  2024-01-01: 0.8117
+  2025-01-01: 0.8191
+  # Trustees' intermediate estimates.
+  2026-01-01: 0.8195
+  2027-01-01: 0.8255
+  2028-01-01: 0.8301
+  2029-01-01: 0.8348
+  2030-01-01: 0.8382
+  2031-01-01: 0.8414
+  2032-01-01: 0.844
+  2033-01-01: 0.8462
+  2034-01-01: 0.8476
+  2035-01-01: 0.8485
+  2040-01-01: 0.8531
+  2045-01-01: 0.8551
+  2050-01-01: 0.8543
+  2055-01-01: 0.8534
+  2060-01-01: 0.8537
+  2065-01-01: 0.8555
+  2070-01-01: 0.8566
+  2075-01-01: 0.8575
+  2080-01-01: 0.8593
+  2085-01-01: 0.8609
+  2090-01-01: 0.863
+  2095-01-01: 0.8638
+  2100-01-01: 0.8639

policyengine_us/parameters/calibration/gov/hhs/medicare/part_d/premiums_per_enrollee.yaml

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+description: The Medicare Trustees' reported and projected Part D premium income and enrollment give these Part D premiums from enrollees per Part D enrollee, excluding the income-related monthly adjustment amount.
+metadata:
+  unit: currency-USD
+  period: year
+  label: Medicare Part D premiums per Part D enrollee, counting retiree drug subsidy beneficiaries as enrollees
+  reference:
+  # Each value is (premium income - aggregate income-related premiums) /
+  # Part D enrollment, from the 2026 Medicare Trustees Report (transmitted
+  # 2026-06-09, retrieved 2026-10-11, sha256
+  # ffa56b9137006872300b0346149eae1613d09a172b6ba118aad48e66dfc48fa8):
+  # - Premium income, Table III.D3 (file page 116), in billions. Footnote 1:
+  #   "Premiums include both amounts withheld from Social Security benefits or
+  #   other Federal payments, direct payments to the Federal government, and
+  #   those paid directly to Part D plans." The table is on a cash basis, and
+  #   footnote 6 (file page 117) credits one January's withheld premiums on
+  #   December 31 of the year before: about $0.2 billion in 2015, $0.1 billion
+  #   in 2020 and an estimated $0.2 billion in 2026. Those amounts are moved
+  #   to the following year: 2016, 2021 and 2027.
+  # - Income-related premiums, Table V.E4 (file page 218), "Aggregate
+  #   premiums", the amount paid "in excess of the Part D plan premium". They
+  #   are part of premium income: they are withheld from benefits (42 U.S.C.
+  #   1395w-113(c)(4)) and deposited into the Part D account (footnote 73,
+  #   file page 151). medicare_cost subtracts each person's own adjustment
+  #   amount instead.
+  # - Part D enrollment, Table V.B3 (file page 196), in thousands. It equals
+  #   the total of Table IV.B7 (file page 153), which includes retiree drug
+  #   subsidy beneficiaries.
+  # Low-income subsidy enrollees pay substantially reduced premiums (file
+  # page 219);
+  # Medicare's low-income subsidy payments are expenditures of the Part D
+  # account, not enrollee premiums (file page 114). So the average is below
+  # the base beneficiary premium.
+  - title: "2026 Annual Report of the Boards of Trustees of the Federal Hospital Insurance and Federal Supplementary Medical Insurance Trust Funds, Tables III.D1, III.D3, IV.B7, V.B3, V.D1 and V.E4 #page 109,114,116-117,151,153,154,196,210,218-219"
+    href: https://www.cms.gov/oact/tr/2026#page=109
+  - title: 42 U.S.C. § 1395w-113(c)(4) - Collection of monthly adjustment amount
+    href: https://www.law.cornell.edu/uscode/text/42/1395w-113#c_4
+values:
+  # Historical.
+  2015-01-01: 277.6
+  2016-01-01: 300.94
+  2017-01-01: 321.53
+  2018-01-01: 316.61
+  2019-01-01: 298.86
+  2020-01-01: 285.44
+  2021-01-01: 306.15
+  2022-01-01: 309.42
+  2023-01-01: 304.39
+  2024-01-01: 307.7
+  # 2025 premiums fell with the voluntary Part D premium demonstration for
+  # stand-alone prescription drug plans (file page 109).
+  2025-01-01: 220.25
+  # Trustees' intermediate estimates. The 2030 jump is the Inflation
+  # Reduction Act's reset of the base beneficiary premium percentage
+  # (file page 154).
+  2026-01-01: 289.79
+  2027-01-01: 373.53
+  2028-01-01: 403.59
+  2029-01-01: 411.27
+  2030-01-01: 637.06
+  2031-01-01: 659.27
+  2032-01-01: 662.52
+  2033-01-01: 684.61
+  2034-01-01: 699.07
+  2035-01-01: 722.05
+  # 2036 to 2100 grow at the 2034-2035 rate (722.05 / 699.07, 3.3% a year),
+  # near the Trustees' 3.1% growth in Part D cost per beneficiary in 2035
+  # (Table V.D1, file page 210); later years hold the 2100 value. See
+  # set_all_uprating_parameters in uprating_extensions.py.

policyengine_us/parameters/uprating_extensions.py

@@ -1181,6 +1181,20 @@ def set_all_uprating_parameters(parameters: ParameterNode) -> ParameterNode:
         growth_years=(2031, 2032),
     )
 
+    # Medicare Part D premiums per Part D enrollee (January values, last
+    # projection year 2035), which medicare_cost reads directly. Each year
+    # from 2036 to 2100 grows at the 2034-2035 rate, near the Trustees' 3.1
+    # percent growth in Part D cost per beneficiary in 2035 (Table V.D1, file
+    # page 210); later years hold the 2100 value. The Part D enrollment share
+    # needs no extension: the Trustees project enrollment through 2100.
+    extend_parameter_values(
+        parameters.calibration.gov.hhs.medicare.part_d.premiums_per_enrollee,
+        last_projected_year=2035,
+        end_year=END_YEAR,
+        period_month=1,
+        period_day=1,
+    )
+
     # CBO income-by-source aggregates are used directly and as anchors for
     # SOI-based income upraters. Extending them in the baseline path keeps
     # long-run nominal data aging independent of scenario-specific reforms.