Calibrate CHIP take-up before estimating premium collections

[daphnehanse11]

Follow-up from PolicyEngine/policyengine-us#8099 and PolicyEngine/policyengine-us#8523.

Problem

PolicyEngine-US currently has state CHIP premium formulas, including tax-unit/family mechanics such as household premiums and family caps. But estimating aggregate CHIP premium collections for #8099 requires a calibrated CHIP-enrolled population, not just modeled eligibility or child counts.

The key work is in policyengine-us-data:

• CPS already exposes current CHIP coverage through NOW_PCHIP.
• policyengine-us-data maps that to reported_has_chip_health_coverage_at_interview.
• CHIP take-up is not currently wired like Medicaid take-up.
• CMS Medicaid/CHIP enrollment raw inputs already include Total CHIP Enrollment by state.

Without CHIP take-up calibration, summing chip_premium by state can overstate collections because takes_up_chip_if_eligible defaults to true in PolicyEngine-US.

Proposed data workflow

1. Use CPS reported_has_chip_health_coverage_at_interview / NOW_PCHIP as a reported-first anchor for CHIP take-up among eligible people.
2. Add CHIP to the take-up generation workflow, analogous to Medicaid where reported coverage anchors are prioritized.
3. Add CMS Total CHIP Enrollment by state as a state-level calibration or diagnostic target.
4. Generate/validate takes_up_chip_if_eligible in enhanced CPS outputs.
5. Aggregate PolicyEngine-US chip_premium by state after CHIP take-up calibration.
6. Produce a comparison table for CT, DE, MA, MI, and NY:
   • CMS Line 49 proxy values from #8099,
   • direct CMS M-CHIP cost-sharing offset rows,
   • calibrated modeled chip_premium aggregate,
   • and #8099 expected ranges.

Cross-repo dependency

PolicyEngine-US may need a small prerequisite input variable for reported CHIP coverage at interview, analogous to existing health coverage inputs such as has_medicaid_health_coverage_at_interview. That companion issue is PolicyEngine/policyengine-us#8523.

The main calibration and estimation work belongs here because it depends on CPS NOW_PCHIP, persisted take-up inputs, CMS enrollment targets, and the enhanced CPS build/calibration pipeline.

Acceptance criteria

• CHIP take-up generation in US-data prioritizes CPS-reported CHIP coverage where available.
• CHIP enrollment is calibrated or at least reported against CMS Total CHIP Enrollment by state.
• Enhanced CPS outputs include usable CHIP take-up inputs for PolicyEngine-US simulations.
• A validation/reporting artifact quantifies calibrated modeled CHIP premium collections by state, especially CT/DE/MA/MI/NY.
• The final premium collection estimate uses tax-unit chip_premium, not premium schedules multiplied by child enrollment counts, so household premiums and family caps are preserved.