Child-support expense prevalence: survey-faithful 0.47% vs retired eCPS's drifted 2-3% vs OCSE-implied ~1.5-2%

The first full-scale run through the child-support signal gate (Build M base, 2026-07-13) surfaced a substantive modeling question the gate band was hiding.

Measured person-level nonzero shares for child_support_expense:

Three observations:

1. Our ASEC channel reproduces the survey exactly; our clone-channel draw (0.27%) is covariate-consistent with the trained marginal. The pipeline is faithful to its source.
2. The retired pipeline inflated prevalence 3–4.5× above the survey marginal it trained on (0.62% → 1.99%/2.96%) — sequential-imputation drift, not a calibrated choice we can find documented. It accidentally landed nearer the OCSE-implied truth.
3. CPS is known to undercount payers (payer self-report). So survey-faithful ≈ undercount; the eCPS number was drifted; admin-aligned would require deliberate seeding.

Why it matters: child-support expense drives SNAP deductions (and other program interactions). Build M will ship payer incidence ~4–6× lower than the eCPS the world used. Neither number is validated.

Proposed resolution (architecture week): treat payer prevalence like the take-up families — seed to an administrative anchor (OCSE payer/caseload counts) with documented source, rather than inheriting either survey undercount or imputation drift. Decision needed on the anchor series and entity mapping.

Immediate mechanical fix (separate PR): the shared (0.005, 0.15) gate band mislabels the faithful 0.466% as degenerate; splitting the expense floor to 0.003 with this evidence as the fence. Per-channel 0.002 degeneracy floors unchanged.

Related: #399 (family restoration), #416 (near-discrete target modeling).

🤖 Generated with Claude Code

Start with the Build M child_support_expense signal gate and compare cps_2024.h5 with the retired extended_cps_2024.h5 and enhanced_cps_2024.h5 results. Review related issues #399 and #416; done means the administrative anchor and entity mapping are decided and documented, with the separate gate-band fix clearly scoped.