Fix duplicate counting in Medicaid non-filer household size #9562

The non-filer branch of medicaid_household_size can count the same person as a spouse, parent, and child-age member. A married couple aged 18 living with their one-year-old child can therefore receive a modeled household size of 6 instead of 3 for each parent, even though everyone belongs to one tax unit.

This occurs without overriding tax_unit_is_filer: with no income and no refundable credits, the model determines that the parents are non-filers. It also occurs with the parent-count inputs omitted, producing 4 instead of 3 for each parent.

Cause and scope

The child-age applicant branch adds:

spouse_count + family_parent_count + family_child_count

These groups overlap. A parent under 19 is also a child-age member, and a spouse can already be included in either family count. The adult branch's separate spouse and child-age counts also need to be checked for overlap when the spouse is child-age.

This is existing shared Medicaid logic, not a bug introduced by the California CCHIP work or the shared income-boundary changes. The overlapping counts are already present in the common base before both PRs.

The implementation is shared across states; the concrete reproductions below were run for California in 2025. An inflated household size increases the poverty guideline used in income comparisons and can affect Medicaid/CHIP eligibility. The zero-income examples isolate the counting error; they do not by themselves demonstrate a change in eligibility.

Reproduction

Save this as a YAML test and run uv run policyengine-core test <file.yaml> -c policyengine_us:

- name: Case 1, non-filing young parents are counted once each.
  period: 2025
  input:
    people:
      person1: {age: 18, own_children_in_household: 1}
      person2: {age: 18, own_children_in_household: 1}
      person3: {age: 1}
    marital_units:
      couple: {members: [person1, person2]}
      child: {members: [person3]}
    households:
      household:
        members: [person1, person2, person3]
        state_code: CA
  output:
    tax_unit_size: 3
    tax_unit_is_filer: false
    medicaid_uses_non_filer_rules: [true, true, false]
    medicaid_household_size: [3, 3, 3]

The input intentionally omits income, filing status, filing propensities, tax_units, families, and spm_units. The structured-input builder puts all three people in one of each omitted group. tax_unit_is_filer: false above is an expected output, not an input override.

Observed failure:

medicaid_household_size@2025: [6. 6. 3.] differs from [3. 3. 3.]

For each parent, the current count is 1 spouse + 2 parents + 3 people under 19 = 6; the correct count is 3 distinct people.

Additional local checks

Six YAML cases were run on the local #9434 worktree with its pending pregnancy-deduplication edits. Filing status was calculated automatically in every case. Array order is parent 1, parent 2, child.

Result: 3 passed, 3 failed. In the pregnancy case, ca_medicaid_household_pregnancies correctly returned [1, 1, 1]; the remaining excess is in the base member count.

Expected correction

• Count each included person once across the applicant, spouse, parent, and child/sibling categories. Deduplicating parent/child overlap alone does not resolve spouse overlap.
• Add YAML regression coverage for the default-filing examples above and spouses represented in separate tax units. Preserve ordinary tax-filer household results.
• Review medicaid_household_income for the corresponding overlapping-member sums so size and income use consistent membership. The six tests above establish the size failures, not the income effects.
• Preserve the distinction between family-group membership and the applicant's actual relationships. The broader relationship-link problem is tracked in Medicaid MAGI household: 'living with both parents' proxy pools grandparent income in multigenerational households #9404.
• Do not work around the counting error by forcing tax_unit_is_filer to true or changing expected outputs to the inflated sizes.

Policy and related work

• 42 CFR 435.603(f)(3): non-filer household composition.
• DHCS MAGI household-size flowchart: Box C identifies the included relatives; steps 7–8 add expected children after household composition is established.
• Related: Medicaid MAGI household: 'living with both parents' proxy pools grandparent income in multigenerational households #9404, which addresses incorrect parent/relative identification in multigenerational households. The duplicate-counting examples here occur within a simple nuclear family.
• Discovered while reviewing Add California County Children's Health Initiative Program (CCHIP) #9434 (California CCHIP), stacked on Correct shared Medicaid and CHIP income boundaries #9561 (shared Medicaid/CHIP income boundaries). This issue tracks a separate follow-up; neither PR fixes the shared base household-size overlap.