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What methods does the US government use to estimate undocumented immigrants?
Executive Summary
The U.S. government principally relies on the aggregate residual method—estimating the total foreign‑born population from surveys, subtracting administratively counted lawful immigrants, and treating the difference as the undocumented population—but it supplements this with administrative‑record and microdata linkage approaches that produce differing estimates and uncertainties [1] [2] [3]. Independent researchers such as Pew and Migration Policy Institute use similar residual frameworks with adjustments for undercounts; recent methodological work also layers administrative‑record integration to capture noncitizens missed by surveys [4] [5] [3].
1. How the Residual Method Frames America’s Unauthorized Count — A Simple Equation, Big Assumptions
The dominant official approach is the aggregate residual method, which starts with survey‑based totals of the foreign‑born population (ACS or CPS) and then subtracts an independently constructed count of lawful immigrants derived from Department of Homeland Security administrative records, adjusted for deaths and emigration; the residual is reported as the unauthorized population [1] [6]. This framing is straightforward to describe but contains critical assumptions about survey coverage, accurate administrative tallies, and correct adjustments for emigration and mortality; any errors in the inputs propagate into the residual. Analysts note that the method requires upward adjustments for known undercounts—especially recent arrivals and young adult men—so the final estimate is less a pure census and more an informed construct combining survey sampling, administrative tallies, and empirical correction factors [7] [4].
2. Alternative and Complementary Methods — Microdata Linkage and Administrative‑Record Hybrids Complicate the Picture
Beyond the aggregate residual, researchers and some government efforts use microdata linkage and administrative‑record synthesis to refine estimates, linking census or survey responses to government records and third‑party datasets to better identify likely noncitizen statuses [2] [3]. These approaches incorporate dozens of administrative sources and create record‑based population estimates that can diverge from traditional survey‑only residuals; proponents argue they better capture nonrespondents and undocumented people who avoid surveys, while critics warn that linkage relies on imperfect identifiers and can misclassify legal statuses. The result is multiple, defensible but different estimates: the residual method yields one signal, administrative‑record syntheses can raise or lower that signal depending on coverage of noncitizen data and inclusion rules [3].
3. Who Uses What and Why the Numbers Don’t Always Match — Institutional Choices Matter
Different institutions—DHS/OHSS, Pew Research Center, Migration Policy Institute, and academic teams—apply variations of the residual approach and administrative adjustments, producing headline differences and sometimes divergent time‑series trends [4] [5] [8]. DHS emphasizes administrative records for the legal counts it subtracts; NGOs and academic groups often apply survey correction factors and modeling choices differently, especially for estimating emigration and undercounts. These institutional choices reflect different priorities and audiences: DHS needs operational breakdowns for policy and enforcement, while research groups prioritize demographic accuracy and international comparison. Where one method treats certain record anomalies as coverage errors, another treats them as evidence of a larger noncitizen presence, producing legitimate methodological disagreement rather than simple error [6] [5].
4. The Biggest Sources of Uncertainty — Undercounts, Emigration, and Status Misclassification
All methods wrestle with three core uncertainties: survey nonresponse and undercounts (especially of recent arrivals and young men), estimating emigration and mortality rates for foreign‑born cohorts, and accurately classifying legal status when records are incomplete or linkage fails [7] [2]. Studies explicitly model upward adjustments for undercounting using Census Bureau research and binational surveys, and administrative approaches may incorporate 31 different record types to reduce unknown status cases; still, each corrective move introduces model dependence. The end product is an estimate with substantive error margins and sensitivity to plausible alternative assumptions, meaning statements like “about X million unauthorized immigrants” reflect a best estimate grounded in contested inputs rather than an exact headcount [4] [3].
5. What the Different Estimates Tell Policymakers — Consistency in Trend, Disagreement in Level and Detail
Despite methodological differences, multiple authoritative estimates converge on broad trends—stability or modest changes in the unauthorized population since the late 2000s—while disagreeing on absolute levels and subgroup breakdowns [6] [8]. Where residual‑based estimates commonly hover around roughly 11 million in recent years, administrative‑record inclusive estimates have at times been higher, reflecting additional noncitizen coverage and classification differences. Policymakers should therefore use these numbers as complementary signals: residual estimates provide a reproducible baseline for trend analysis, while administrative and microdata syntheses reveal potential undercoverage and population heterogeneity that matter for enforcement, services, and demographic research [4] [3].