September 1, 2010

U.S. Unauthorized Immigration Flows Are Down Sharply Since Mid-Decade

Appendix C: Methodology

Unauthorized Immigrants—Overview

The data presented in this report on unauthorized and legal immigrants were developed through a multistage estimation process, principally using March Supplements to the Current Population Survey (CPS). The CPS is a monthly survey of about 55,000 households conducted jointly by the U.S. Bureau of Labor Statistics and the Census Bureau; the sample is expanded to about 80,000 households for the March supplement.

The first stage in the estimation process uses CPS data as a basis for estimating the number of legal and unauthorized immigrants included in the survey and the total number in the country using a residual estimation methodology. This method compares an estimate of the number of immigrants residing legally in the country with the total number in the CPS; the difference is assumed to be the number of unauthorized immigrants in the CPS. The legal resident immigrant population is estimated by applying demographic methods to counts of legal admissions covering the period from 1980 to the present obtained from the Department of Homeland Security’s Office of Immigration Statistics and its predecessor at the Immigration and Naturalization Service. The initial estimates here are calculated separately for age-gender groups in six states (California, Texas, Florida, New York, Illinois and New Jersey) and the balance of the country; within these areas the estimates are further subdivided into immigrant populations from 35 countries or groups of countries by period of arrival in the United States. Variants of the residual method have been widely used and are generally accepted as the best current estimates. See also Passel and Cohn 2008; Passel 2007 for more details.

Then, having estimated the number of legal and unauthorized immigrants in the March CPS Supplements, we assign individual foreign-born respondents in the survey a specific status (one option being unauthorized immigrant) based on the individual’s demographic, social, economic, geographic and family characteristics. (See below for more details.) The data and methods for the overall process were developed initially at the Urban Institute by Passel and Clark (especially 1998) and were extended by work of Passel, Van Hook and Bean (2004) and by subsequent work at the Pew Hispanic Center.

The final step adjusts the estimates of legal and unauthorized immigrants counted in the survey for omissions. The basic information on coverage is drawn principally from comparisons with Mexican data, U.S. mortality data and specialized surveys conducted at the time of the 2000 Census (Bean et al. 1998; Capps et al. 2002; Marcelli and Ong 2002). These adjustments increase the estimate of the legal foreign-born population, generally by 1-3% and the unauthorized immigrant population by 10-15%. The individual survey weights are adjusted to account for immigrants missing from the survey. These augmented files serve as a basis for the detailed tabulations of the family, social, economic and geographic characteristics presented here.

Status Assignments—Legal and Unauthorized Immigrants

Individual respondents are assigned a status as a legal or unauthorized immigrant based on the individual’s demographic, social, economic and geographic characteristics so the resulting number of immigrants in various categories agrees with the totals from the residual estimates. The assignment procedure employs a variety of methods, assumptions and data sources.

First, all immigrants entering the United States before 1980 are assumed to be legal. Then, the CPS data are corrected for known over-reporting of naturalized citizenship on the part of recently arrived immigrants (Passel et al. 1997) and all remaining naturalized citizens from countries other than Mexico and those in Central America are assigned as legal. Persons entering the U.S. as refugees are identified on the basis of country of birth and year of immigration to align with known admissions of refugees and asylees (persons granted asylum). Then, individuals holding certain kinds of temporary visas (including students, diplomats and “high-tech guest workers”) are identified in the survey, and each is assigned a specific legal temporary migration status using information on country of birth, date of entry, occupation, education and certain family characteristics. Finally, some individuals are assigned as legal immigrants because they are in certain occupations (e.g., police officer, lawyer, military occupation, federal job) that require legal status or because they are receiving public benefits (e.g., welfare or food stamps) that are limited to legal immigrants.

After these initial assignments as “definitely legal” immigrants, a pool of “potentially unauthorized” immigrants remains. This group typically exceeds the target residual estimates by 20-35%. The “potentially unauthorized” immigrants are assigned as legal or unauthorized with probabilistic methods. This last step involves checks to ensure consistent statuses within families and several iterations to reach agreement with the demographically derived population totals.

At the end, the final estimates agree with the residual estimates for the six individual states noted earlier and for the balance of the country; for Mexican-born and other legal and unauthorized immigrants in each area; and for children, working-age men and working-age women within each category. Finally, the survey weights for foreign-born individuals are adjusted upward so the tabulated figures agree with the analytic, demographic estimates of the total number of legal and unauthorized migrants developed in the very first step. The end product is a survey data set (of about 80,000 households) with individual respondents identified by nativity and legal status.

This methodology obviously requires a number of assumptions and is applied to survey data from a sample (albeit a large one). The resulting estimates, such as those presented here, are subject to both sampling and non-sampling error. Accordingly, small differences should not be treated as significant or substantive. Sampling error intervals have been developed for the national estimates of all unauthorized immigrants, totals by country or region of birth, and state-level estimates.

CPS Weights

Population data from the CPS are tied to the Census Bureau’s official population estimates of the civilian, noninstitutional population for the nation and states through a weighting process designed so that the CPS figures agree with pre-specified national population totals by age, sex, race and Hispanic origin and with state-level totals by age, sex and race (U.S. Census Bureau 2006). At the end of each calendar year, the Census Bureau produces an estimate of the population of the United States and states for the middle of that calendar year (July 1). The estimate updates the population enumerated in the previous census using the latest available data on demographic components of change. So, in December 2008, the Census Bureau estimated the U.S. population as of July 1, 2008, by updating the census count of April 1, 2000, and taking into account the number of births over those eight years, the number of deaths, and net international migration since 2000. In the course of producing this estimate, the Census Bureau also produces estimates for each month from May 2000 through June 2008. This series of population estimates is referred to by the Census Bureau as the “Vintage 2008” population estimates. The Census Bureau then uses these estimates as a basis for projecting the population forward through the next calendar year (in this case, 2009). These short-term projections serve as the basis for the CPS weights throughout the calendar year. Thus, the weights for each month of the 2009 CPS are based on the Vintage 2008 population estimates; those for the 2008 CPS on the Vintage 2007 population estimates; etc.

For most years, any changes in the series of population estimates from one vintage to the next are small—reflecting mainly the incorporation of final data on births, deaths and immigration for the preliminary data used the year before. However, in the 2007 and 2008 population estimates, the Census Bureau made significant changes in the methodology used to measure international migration from 2000 onward. Although these changes do not directly affect the measured size of the immigrant population, they are concentrated in groups where a high percentage of the population is foreign born, notably working-age Hispanics and Asians. As such, the new population controls have the potential for affecting the measured size of the foreign-born population.

Unfortunately for data users, the Census Bureau rarely reweights the CPS data series to take into account changes in the population estimates across vintages.1 The Census Bureau issued revised weights for 2000-2002 to incorporate large changes engendered by the replacement of the updated 1990 Census with results from the 2000 Census. Because of the large change between the Vintage 2006 and 2007 estimates noted here, the Census Bureau revised CPS weights for research purposes, but for only one month of data—December 2007.

Although the changes caused by reweighting are relatively small as a share of the population, their impact can be relatively greater on the residual estimates of unauthorized immigrants. These methodological changes led to a reduction of about 1.1 million in the estimated population for March 2007 between the Vintage 2006 estimates and the Vintage 2008 estimates. Although this change represented only about 0.4% of the U.S. population, it was concentrated in the Hispanic and Asian populations because immigration plays such a large role in these groups. The differences were further concentrated in adult age groups so that the impact on the Hispanic population was about 1.5%, with some age groups being more than 2% smaller in the Vintage 2008 population estimate than the previous one. As a result, there is a major discontinuity between the CPS results for 2007 and earlier compared with those for 2008 and later.

Although the changes caused by reweighting are relatively small as a share of the population, their impact can be relatively greater on the residual estimates of unauthorized immigrants. These methodological changes led to a reduction of about 1.1 million in the estimated population for March 2007 between the Vintage 2006 estimates and the Vintage 2008 estimates. Although this change represented only about 0.4% of the U.S. population, it was concentrated in the Hispanic and Asian populations because immigration plays such a large role in these groups. The differences were further concentrated in adult age groups so that the impact on the Hispanic population was about 1.5%, with some age groups being more than 2% smaller in the Vintage 2008 population estimate than the previous one. As a result, there is a major discontinuity between the CPS results for 2007 and earlier compared with those for 2008 and later.

State-level Estimates

State-level estimates should be treated with some caution because they are based on much smaller samples than the national estimates. For 2008 and 2005, the estimates are generally averages across three years of data (2006-2008 and 2004-2006) with some estimates based on regression analyses. Ranges of error for the 2008 estimates are based on regression analyses of data for 2000-2008 and CPS standard errors.

The estimates presented for states in 2009 are based on tabulations of the augmented March 2009 CPS file. Because of the change in trend after 2007, there was no appropriate choice for averaging across years to reduce potential measurement error. This choice is reflected in expanded ranges for some states.

Rounding of Estimates. All state-level estimates for unauthorized immigrant populations are presented as rounded numbers to avoid the appearance of unwarranted precision in the estimates. No estimates smaller than 10,000 are shown. Estimates in the range of 10,000-100,000 are rounded to the nearest 5,000; estimates in the range of 100,000-250,000 to the nearest 10,000; estimates smaller than 1 million to the nearest 25,000; and estimates larger than that to the nearest 50,000. The same rounding conventions are applied to all state-level estimates of unauthorized immigrant populations and labor force for 2000 and later and, more generally, to most of the data presented on unauthorized immigrants.

Country of Birth

Some modifications in the original CPS countries of birth were introduced to ensure that all foreign-born respondents could be assigned to a specific country or region of birth. See Passel and Cohn (2008) for a detailed treatment of how persons with unknown country of birth were assigned to specific countries.

The estimates of the unauthorized population shown in this report divide the world into regions. “Latin America” is defined to include Mexico, Central America, Caribbean countries and South America. “Europe” includes Russia and all of the newly independent countries that were part of the former Soviet Union, even though some of the countries are geographically in Asia. This grouping is designed to maintain maximum consistency over time and with the administrative data series used. While all of these countries are separately identified in immigration statistics since their independence, they do not appear in immigration statistics of the 1980s and most are not identified as countries of birth in the CPS. “Middle East” as defined here includes countries of southwest Asia from Turkey and Cyprus in the north and west to Iran in the east to the Arabian Peninsula in the south; it also includes countries of North Africa (Egypt, Sudan, Libya, Tunisia, Algeria, Morocco and Western Sahara). Note that the Middle East does not include Afghanistan or Pakistan. “South and East Asia” is the rest of Asia from Afghanistan and Pakistan eastward. “Other” consists of sub-Saharan Africa and Oceania; in addition, the few respondents not assigned to any other areas are categorized as being from “Other.”

Estimates of Migration Flows

The estimates of unauthorized immigrants measure the number of unauthorized immigrants in the country at different point in time; they do not directly measure the number coming into the country in a year or the number leaving the country.

The residual methodology does provide estimates of the number arriving in five-year periods from 1980 to the estimate date. Similarly, tabulations of the CPS data with status assignments provide alternative estimates of arrivals in two-year periods beginning with 1980 and a final period of slightly more than two years or slightly more than three years ending at the survey date. Differences in the size of arrival cohorts based on these alternative measures can be used to assess inflows and outflows of unauthorized immigrants for shorter intervals, especially for one-year periods from March of one year to March of the next. The estimates shown in this report for inflows of unauthorized immigrants are averages of estimates based on tabulations of augmented March CPS datasets and the underlying residual estimates. A more detailed exposition of the methodology used can be found in Passel and Cohn 2009a.

  1. numoffset=”7″ The Census Bureau issued revised weights for 2000-2002 to incorporate large changes engendered by the replacement of the updated 1990 Census with results from the 2000 Census. Because of the large change between the Vintage 2006 and 2007 estimates noted here, the Census Bureau revised CPS weights for research purposes, but for only one month of data—December 2007.