Rules-based Marketplace net premium and Medicare Part B for reform-responsive MOOP

[MaxGhenis]

Part 3/4/5 of the architecture described in #8089. Filed as a dedicated issue because ACA has the largest MOOP magnitude among the rules-computable premium components.

Same shape as CHIP

PolicyEngine-US already computes premium_tax_credit (the subsidy). What's missing is a net Marketplace premium:

marketplace_net_premium = silver_benchmark
                         × applicable_figure_of_income
                         × max(0, tax_unit_income − applicable_contribution_basis)
                         − premium_tax_credit

(Or more simply, applicant-contribution-based net premium for the benchmark silver plan.)

The CPS-imputed health_insurance_premiums_without_medicare_part_b in PolicyEngine-US already includes whatever a Marketplace enrollee reported paying post-PTC. So on a baseline run, MOOP is roughly right. But a PTC reform (e.g., eliminate the enhanced PTC, shift applicable-figure curves) changes computed PTC while imputed MOOP stays put, and SPM resources don't move.

Same swap the CHIP side needs:

health_insurance_premiums = imputed_residual
                          + computed_marketplace_net_premium
                          + computed_medicare_part_b_premium
                          + computed_chip_premium

with imputed_residual = imputed_total − baseline_computed_sum (baseline computed at the CPS reference year's rules, 2024, mirroring the CHIP approach).

What's needed, roughly

1. Rules-based marketplace_net_premium variable in policyengine-us.
   • Silver benchmark × applicable-figure formula.
   • Gated on Marketplace enrollment (new flag or use an existing "does this tax unit buy on Marketplace" indicator; CPS has Marketplace coverage flag).
   • Tax-unit or SPM-unit entity.
2. Calibration for Marketplace premium revenue per enrollee by state, sourced from HHS Marketplace effectuated enrollment + issuer premium data (or simpler, use average benchmark × applicable-figure at tax-unit-MAGI).
3. MOOP swap in cliff-watch (and eventually in the baseline us-data imputation) to avoid double-count.

Why this matters for cliff-watch specifically

ACA subsidy cliffs are famously large — the subsidy-cliff removal through the ARPA/IRA enhanced PTC is a policy reform itself, so users simulating a return to pre-ARPA rules want the net-income impact to show up. With CPS-imputed MOOP alone, that reform shows up only as a premium_tax_credit drop on the benefits side, not as the corresponding premium spike on the costs side. Users see half the cliff.

Medicare Part B as well

The same follow-up should set up computed Medicare Part B:

medicare_part_b = base_premium + IRMAA_surcharge(MAGI, filing_status)

Gated on Medicare enrollment (age ≥ 65 or disability-eligible). Current PolicyEngine-US uses medicare_part_b_premiums as a pure CPS-imputed input. Rules-based would respond to income cliffs (IRMAA tier crossings at MAGI thresholds) and to reforms to IRMAA breakpoints.

Suggested sequencing

1. Rules-based medicare_part_b_premium variable (cleanest — single statutory formula).
2. Rules-based marketplace_net_premium variable (requires Marketplace enrollment flag; more data work).
3. household_health_costs list in policyengine-us (parallel to the existing household_health_benefits; extends Net chip_premium from household_net_income #8088).
4. us-data baseline-year computed subtraction from imputed MOOP.
5. cliff-watch consumes new variables.

Related

• Add chip_premium with 17-state CHIP premium implementations #8086 — CHIP premium framework
• Net chip_premium from household_net_income #8088 — household_net_income subtract
• MOOP should use computed premiums instead of fully imputed, so reforms propagate to SPM #8089 — overarching MOOP architecture
• Include chip_premium in SPM expenses so reforms propagate #8090 — CHIP premium into SPM expenses
• Encode historical MO and NY CHIP premium schedules for 2020-2026 #8091 — 2024 CHIP rule history
• Reconstruct gross CHIP per-capita value for premium netting #8094 — gross CHIP reconstruction