How do CPS and ACS estimates of the foreign‑born population differ and why does it matter for policy?
Executive summary
The CPS and ACS produce different headline counts and recent trends for the U.S. foreign‑born population because they differ in sample size, frequency and reference dates, survey mode and weighting, and how they handled pandemic-era disruptions — differences that matter for everything from national immigration debates to state and local resource planning [1] [2] [3]. Analysts and advocacy groups have exploited these methodological gaps: some favor the monthly CPS as more up‑to‑date while others point to the ACS’s larger sample and tighter margins of error, so policymakers must treat both as imperfect but complementary signals [1] [4] [5].
1. How the surveys differ at a glance: size, timing and questions
The American Community Survey (ACS) is a much larger, continuous monthly survey tabulated into annual estimates and designed to replace decennial sample data, producing more precise subnational and subgroup estimates than the CPS, which has a much smaller sample and is conducted monthly primarily for labor statistics [1] [6]. The CPS ASEC supplement provides an annual snapshot (commonly referenced by March) while ACS estimates reference mid‑year (commonly July), so simple year‑to‑year comparisons can reflect different time windows rather than true disagreement about levels [1] [3] [2]. The CPS and ACS ask about place of birth and citizenship in comparable ways, but the CPS historically asked additional parental nativity questions that ACS no longer collects, creating differences for intergenerational analysis [1].
2. Why counts and trends diverge: precision, margins and pandemic noise
Because the ACS’s larger sample yields smaller margins of error, it tends to give more statistically precise estimates for states and detailed subgroups, whereas CPS estimates fluctuate more from month to month and have larger sampling error — a feature that can magnify perceived recent growth when analysts focus on CPS monthly totals [1] [7]. Recent years illustrate this: ACS data implied about 1.9 million net migration from mid‑2022 to mid‑2023 while CPS ASEC-based accounting suggested roughly 2.8 million over a different annual window, a gap magnified by differing reference dates and sampling noise [7] [8]. Pandemic-era disruptions and lower response rates in 2020 add another layer of uncertainty: the ACS acknowledged quality issues in 2020 while the BLS defended CPS estimates but warned that changes in response patterns could bias trends [5].
3. Coverage and undercounts: who’s missing matters for policy
Both surveys undercount some foreign‑born groups, particularly unauthorized immigrants, and do not capture immigration status directly in most public microdata, complicating estimates of services needed or fiscal impacts; researchers warn that subgroup sums can exceed totals because of measurement and adjustment differences [9] [10]. The CPS excludes the institutionalized population in some tabulations, a divergence from ACS that can alter state or local counts where institutional facilities are significant [4]. Weighting and stratification choices — CPS sampling strata are geography- and socioeconomic-driven, not explicitly calibrated to foreign‑born distributions — mean regional over‑ or underrepresentation can occur across months [1].
4. Why the differences matter for policy and politics
At the national level, headline differences feed competing narratives: groups pressing for tougher border controls cite CPS’s larger recent increases to argue for urgency, while others point to ACS’s precision and different timing to temper claims of runaway migration [3] [8]. For labor market, health, education and fiscal planning, the ACS’s precision at local levels makes it the preferred input for program budgeting and infrastructure planning, but its lagged reference can miss very recent surges that CPS’s monthly cadence better captures — creating a tradeoff between timeliness and statistical certainty [1] [2]. Because both surveys undercount certain populations, policymakers should avoid overreliance on a single series for hard resource allocations and instead triangulate with administrative records and sensitivity analyses [10] [9].
5. How to use both surveys prudently
A pragmatic approach treats the ACS as the backbone for state/local estimates and subgroup detail, uses CPS for near‑real‑time signals and labor force context, and explicitly quantifies margin‑of‑error and timing differences when forming policy conclusions; analysts should disclose known data limitations such as the 2020 quality impacts and potential nonresponse bias among immigrants [1] [5] [2]. Where advocacy groups or think tanks highlight one source over another, readers should note institutional agendas — some prioritize timeliness to press political points while others emphasize statistical precision — and demand transparent methods and sensitivity checks [4] [8].