Source external benchmarks for CTC child-composition calibration

[MaxGhenis]

Recent CTC follow-up work now has:

• direct ID-model follow-up tracked in Model TIN status separately from ssn_card_type in ECPS #712
• AGI/filing-status CTC component targets tracked in Add AGI-by-filing-status CTC targets for refundable and nonrefundable components #717
• child-composition diagnostics/validation tracked in Add child-composition CTC targets or validation #725

That still leaves one distinct gap: we do not currently have any external benchmark source for the CTC child composition that seems to be driving reform sensitivity.

Why this needs its own issue

I compared adjacent staged national ECPS artifacts after the recent CTC work and found that total ctc_qualifying_children moved only modestly, but the composition moved much more:

• total ctc_qualifying_children: about -570k
• tax units with 1 qualifying child: about -1.50M
• tax units with 2 qualifying children: about -1.34M
• tax units with 3+ qualifying children: about +1.06M
• under-6 qualifying children: about -341k
• age 6-17 qualifying children: about -229k

The biggest shifts were compositional inside low/mid AGI bands and filing-status cells, for example:

• Head of household, $25k-$50k: about -3.21M weighted qualifying children
• Head of household, $10k-$25k: about +3.12M
• Joint / surviving spouse, $10k-$25k: about -2.31M
• Joint / surviving spouse, $1-$10k: about +1.80M

That is exactly the kind of movement that can leave aggregate current-law totals looking acceptable while still moving ARPA-style or AFA-style reform effects materially.

Problem

The repo currently has the machinery to diagnose child composition, but not to target it against an external benchmark:

• we can report ctc_qualifying_children
• we can report 0 / 1 / 2 / 3+ qualifying-child buckets
• we can report under-6 vs age-6-to-17 qualifying-child buckets
• but we do not have source-backed target tables for those quantities

The current IRS/SOI ETL only gives us aggregate CTC amount/count targets and some AGI splits. It does not provide child-count or child-age composition targets for CTC qualifying children.

Proposal

Track a data-sourcing effort to identify the best available benchmark for at least some of these CTC-relevant child-composition quantities:

• total ctc_qualifying_children
• tax-unit distribution by 0 / 1 / 2 / 3+ CTC qualifying children
• under-6 vs age-6-to-17 CTC qualifying children
• ideally the same crossed with AGI band and/or filing status where possible

Potential source classes:

• IRS tabulations not yet wired into the repo
• CTC-focused Treasury / JCT / CRS tabulations
• high-quality survey-admin linked external research if direct IRS publication tables do not exist

Success criteria

• We identify whether a defensible external benchmark exists for any of the high-value child-composition buckets.
• If it does, we add ETL/targets for those buckets.
• If it does not, we document that clearly so Add child-composition CTC targets or validation #725 remains validation-only rather than an implicit calibration promise.

[MaxGhenis]

I found defensible public-source paths for most of the child-composition problem, but not for the full 0/1/2/3+ qualifying-child distribution.

Best public-source options I found:

• Total qualifying children

  • IRS Publication 4801 line-item estimates for Schedule 8812 (2022), especially line 4
  • This gives a direct admin count of qualifying children under age 17 with required SSNs

• Total qualifying children plus under-6 vs age-6-to-17 split

  • IRS Publication 4801 line-item estimates for Schedule 8812 (2021)
  • This gives direct admin counts for total qualifying children, under age 6, and ages 6-17
  • Caveat: the clean public line-item age split appears in 2021; I did not find the same clean public split for 2022

• Qualifying children by AGI

  • IRS SOI “Advance Child Tax Credit payments in 2021” Table 1 (21in01actc.xls)
  • This reports number of qualifying children by AGI band

• Qualifying children by filing status

  • IRS SOI “Advance Child Tax Credit payments in 2021” Table 3 (21in03actc.xls)
  • This reports number of qualifying children by filing status

What I did not find:

• a public IRS table giving the 0 / 1 / 2 / 3+ distribution of CTC qualifying children
• a public IRS table giving the under-6 split and the full AGI/filing-status cross-tabs together

So the practical implication is:

• total qualifying children looks sourceable from direct IRS admin data
• under-6 vs 6-17 looks sourceable, but likely only from the 2021 Schedule 8812 line-item publication path
• by AGI and by filing status look sourceable from the 2021 advance CTC admin tables
• 0/1/2/3+ still looks like the real gap

Recommendation:

• Use the public IRS sources above to strengthen calibration/validation on totals, age split, AGI mix, and filing-status mix
• Treat the 0/1/2/3+ child-count distribution as a separate proxy/constructed-benchmark problem unless we find a non-public or quasi-admin source that supports it directly

[MaxGhenis]

Small correction from the follow-up source check:

• The clean current-law-ish total qualifying-child benchmark from Publication 4801 is the 2022 amounts table for Schedule 8812 line 4, which gives 63.622 million qualifying children under age 17 with required SSNs.
• The 2022 returns-filed table value (37.274 million) is the number of returns with a line-4 entry, not the number of qualifying children.
• Likewise for the 2021 age-split Schedule 8812 source: the returns-filed page gives return counts, not child counts. The amounts page gives the aggregate child counts.

That leaves the source picture as:

• exact total ctc_qualifying_children: yes, from 2022 Publication 4801 amounts table
• age split: only from 2021 Schedule 8812 amounts table, and that is an ARPA under-18 concept rather than a clean current-law under-17 concept
• AGI and filing-status mix: yes, from the 2021 advance CTC admin tables, but again as a contextual benchmark rather than an exact current-law target

So I’m comfortable using:

• the 2022 Publication 4801 total as a direct validator reference
• the 2021 advance-CTC AGI / filing-status distributions as contextual share comparisons
• but not the 2021 under-6 / 6-to-17 split as a hard current-law validation target