MOOP should use computed premiums instead of fully imputed, so reforms propagate to SPM

[MaxGhenis]

Follow-up from #8086 and #8088.

Problem

Medical out-of-pocket expenses (MOOP) are used to compute SPM resources:

spm_unit_net_income = market_income + benefits − taxes − spm_expenses
spm_expenses         = child_support + medical_out_of_pocket_expenses + capped_childcare
medical_out_of_pocket_expenses = health_insurance_premiums + other_medical_expenses

health_insurance_premiums (and its Medicare-Part-B split) are pure CPS-imputed inputs — no formula, just survey data uprated via calibration.gov.hhs.cms.moop_per_capita:

• policyengine_us/variables/household/expense/health/health_insurance_premiums_without_medicare_part_b.py
• policyengine_us/variables/household/expense/health/medicare_part_b_premiums.py

This means:

• Baseline SPM is fine: imputed MOOP already includes whatever CHIP / Medicare / Marketplace premiums CPS respondents reported.
• Reforms break: if a reform eliminates CHIP premiums, simulated chip_premium drops to zero, but MOOP (survey-imputed) doesn't change, so SPM resources don't respond. Same for Medicare Part B IRMAA reforms, Marketplace benchmark changes, etc.

Proposed architecture

Whatever we compute from policy rules should replace (not add to) the corresponding slice of imputed MOOP:

medical_out_of_pocket_expenses = imputed_moop_residual
                                + computed_chip_premium
                                + computed_medicare_part_b_premium
                                + computed_marketplace_net_premium
                                + other_computed_premiums...

where

imputed_moop_residual = imputed_moop_total - baseline_computed_premium_sum

baseline_computed_premium_sum is pre-computed per CPS record during data construction using the rules that were in effect during the CPS reference year (currently 2024). In baseline simulations the two cancel; in reforms the computed piece moves and SPM adjusts correctly.

What's computable today

Program	Can we compute it from rules?	Status
CHIP premiums	Yes — #8086	chip_premium (17 states)
Medicare Part B	Yes — statutory base + IRMAA tiers by MAGI	Not computed
Marketplace net premium	Yes — silver benchmark × applicable-fig percent of income − PTC	premium_tax_credit computed; net premium not assembled
Medicaid cost-sharing	Partial — varies widely by state	Out of scope
Copays / deductibles	No — not rule-driven	Stay in imputed residual

Staging

1. us-model (this issue / follow-up PR) — add chip_premium to the MOOP chain so reforms to CHIP premiums propagate to SPM. Accepts a small baseline double-count until step 2 lands.
2. us-data — during data construction, compute each CPS record's CHIP premium under 2024 rules (the reference year) and subtract it from the imputed health_insurance_premiums_without_medicare_part_b component. After this, baseline reconciles exactly.
3. us-model (later) — rules-based Medicare Part B (standard premium + IRMAA by MAGI tier), plug into same MOOP chain.
4. us-data (later) — strip Medicare Part B from imputed MOOP using 2024 rules.
5. us-model / us-data (later) — rules-based Marketplace net premium using existing premium_tax_credit plus benchmark × affordability percent.

Notes

• The "2024 rules" requirement for step 2 comes from the CPS reference year. As the dataset updates (CPS ASEC 2026 → reference year 2025 etc.), the anchor year updates too.
• Entity-mismatch caveat: chip_premium is TaxUnit-level, medical_out_of_pocket_expenses is Person-level. Aggregation goes through add() at the SPM unit level (existing precedent: spm_unit_benefits adds TaxUnit-level premium_tax_credit). Architecturally clean at the SPM-unit boundary.

[MaxGhenis]

Post-merge status update:

Several model-side pieces from the original staging list are now present on main:

• chip_premium exists and is aggregated through spm_unit_medical_out_of_pocket_expenses.
• income_adjusted_part_b_premium is used in the SPM MOOP wrapper after subtracting imputed medicare_part_b_premiums.
• marketplace_net_premium exists after Fix unsubsidized Marketplace net premiums #8163, and household_health_costs includes it for net income with health benefits.
• medicaid_premium exists as a federal aggregator with Indiana HIP currently wired.

The remaining architecture gap is still the important one: us-data needs a baseline computed-premium residualization path, so baseline imputed MOOP does not double-count computed CHIP / Part B / Marketplace / Medicaid premium components while reforms remain responsive.

Related remaining trackers: #8095 for Marketplace/Part B MOOP follow-through, #8168 for Marketplace take-up and selected-plan ratio calibration, #8101 for the CHIP 5% cap, and #7972/#7967 for Medicare Part B MSP cleanup.