What methodologies produce the large disparities between FAIR’s 18.6M estimate and Pew’s 14M estimate of unauthorized residents?
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Executive summary
The gulf between FAIR’s 18.6 million figure and Pew’s 14 million estimate of unauthorized residents flows from methodological choices about data sources, timing, who is counted as “unauthorized,” and how analysts correct for survey omissions and population revisions; Pew’s public documentation emphasizes a residual method built on Census surveys and demographic adjustments, while the materials supplied do not include FAIR’s technical methods so direct attribution to FAIR’s procedures is limited [1] [2] [3]. Below is a focused breakdown of the principal methodological levers that routinely produce multi‑million differences between competing estimates, anchored in Pew’s published practices and the broader literature on residual estimation.
1. Data source and benchmark population: which survey or administrative series anchors the count
Pew’s 2023 estimate is explicitly built on the American Community Survey (ACS) and Census Bureau population benchmarks, using the ACS as the primary detailed survey input and aligning with Census Bureau vintage revisions that added roughly 2 million immigrants in their 2024 population update—an adjustment Pew says materially increased its recent unauthorized total [1] [4] [5]. The choice of ACS versus other surveys or administrative records matters because each source has different coverage patterns and timing; analysts using a survey with lower estimated coverage of recent arrivals or excluding the Census vintage revision will produce a lower or higher residual estimate [2] [4].
2. The residual method’s definitional and arithmetic choices
Pew uses a residual methodology: estimate total foreign‑born from surveys, separately estimate the legally resident foreign‑born by adding admissions and subtracting deaths and departures, and treat the difference as the unauthorized population—steps that require many assumptions and imputations and that Pew documents in Methodology A and B [2] [6]. Small differences in imputations—how many temporary legal residents are projected, how emigration rates are modeled, or how deaths are applied—compound across millions of records; the SSA review and methodological literature note that imputation rules and the particular residual variant used explain why Pew, DHS, MPI and CMS sometimes diverge [3].
3. Adjustments for survey omissions, undercount and weighting
Pew explicitly adjusts ACS/CPS-based estimates for survey omissions and known errors and cautions that such corrections change totals and time trends; the Center’s estimates therefore can rise or fall when census population controls are revised [2] [5]. Any alternative estimate that applies different omission corrections—either smaller or larger upward adjustments for hard‑to‑count populations—will differ by millions because the unauthorized population is concentrated among households more likely to be missed or misclassified in surveys [2] [6].
4. Treatment of temporary protections and ambiguous legal statuses
Pew’s public notes emphasize that many people in their “unauthorized” residual may actually have temporary protections or specific non‑deportable statuses and that analysts must decide whether to include such groups and how; Pew’s 2022 estimate explicitly included millions with temporary protection from deportation in its residual accounting [7] [1]. Differing decisions about whether to classify DACA recipients, TPS holders, recent asylum applicants, or certain guest‑worker permutations as “unauthorized” or “lawful temporary” can shift estimates substantially [7] [2].
5. Reference date, rounding, and revisions to net international migration
Pew’s 14 million is a point estimate for July 2023 and incorporates Census Bureau vintage revisions that reestimated net international migration—revisions the Center says added about 2 million immigrants to the population baseline, almost all unauthorized arrivals since 2021 [1] [4]. Competing counts that use a different reference date, do not incorporate the Vintage 2024 revision, or apply different rounding/aggregation rules will therefore diverge; Pew warns that large revisions to baseline population inputs create “major discontinuities” in trend series [4].
6. What can’t be settled from the supplied reporting — FAIR’s technical choices
The supplied sources comprehensively document Pew’s residual approach and the literature on residual estimation [1] [2] [3], but they do not include FAIR’s underlying datasets, imputation rules, or published methodology in the materials provided here; therefore it is not possible, on the basis of these sources alone, to attribute the 4.6 million difference specifically to a single procedural decision by FAIR. To resolve the gap empirically requires comparing FAIR’s technical appendix (if published) against Pew’s Methodology A/B line‑by‑line—examining data inputs, inclusion/exclusion rules for temporary statuses, omission‑correction magnitudes, and the chosen reference date [2] [4] [6].