Problem

After #8163, PolicyEngine-US computes marketplace_net_premium for subsidized and unsubsidized Marketplace buyers, but two inputs remain coarse:

• takes_up_aca_if_eligible is a tax-unit-level default-true switch used to gate Marketplace premium payment, including unsubsidized buyers.
• selected_marketplace_plan_benchmark_ratio defaults to 1.0, so the selected plan is assumed to cost the same as the SLCSP benchmark unless the user supplies an override.

This is workable for policy mechanics, but it limits baseline calibration and reform analysis. Actual Marketplace enrollees choose bronze, silver, gold, and other plans with selected premiums that can differ substantially from the benchmark. The selected premium determines marketplace_net_premium, which feeds health-cost accounting and eventually MOOP/SPM work in #8089 and #8095.

Desired direction

Define the model/data contract for observed Marketplace coverage and selected plan cost:

• Decide whether has_marketplace_health_coverage_at_interview should inform baseline Marketplace take-up, while preserving a reform-responsive simulated take-up path.
• Replace the unconditional selected_marketplace_plan_benchmark_ratio = 1.0 default with either a calibrated/imputed ratio or a documented explicit-input path.
• Consider calibration by state, income/FPL band, metal tier, age/family structure, or a simpler national/state average if the data cannot support richer variation.
• Keep the result compatible with marketplace_net_premium, used_aca_ptc, and future MOOP residualization.

Acceptance criteria

• Documented decision on observed vs simulated Marketplace take-up.
• A calibrated or explicitly user-supplied treatment of selected_marketplace_plan_benchmark_ratio.
• Tests covering a benchmark-plan case, a lower-premium selected plan (e.g. bronze-like ratio below 1), and an unsubsidized Marketplace buyer.
• Cross-reference or integration note for MOOP should use computed premiums instead of fully imputed, so reforms propagate to SPM #8089 and Rules-based Marketplace net premium and Medicare Part B for reform-responsive MOOP #8095 so MOOP residualization uses the same selected-premium concept.

Related

• Fix unsubsidized Marketplace net premiums #8163 — added unsubsidized Marketplace net premium support.
• Rules-based Marketplace net premium and Medicare Part B for reform-responsive MOOP #8095 — Marketplace net premium and Medicare Part B for reform-responsive MOOP.
• MOOP should use computed premiums instead of fully imputed, so reforms propagate to SPM #8089 — broader computed-premium residualization architecture.
• Complete is_aca_eshi_eligible by modeling disqualifying ESI offers #5963 — ACA eligibility gap around disqualifying ESI offers.