August 11, 2010

Unauthorized Immigrants and Their U.S.-Born Children

Appendix B: 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 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, described in detail in previous reports and papers (especially Passel and Cohn 2008; Passel 2007).

Then, the March CPS Supplements are augmented to assign a specific legal status (one option is unauthorized immigrant) to each foreign-born survey respondent and to adjust the survey weights 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. 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, Bean and Van Hook (2004) and by subsequent work at the Pew Hispanic Center.

The residual method compares the number of immigrants in the CPS with an independently derived demographic estimate of legal foreign-born residents. The difference between the survey total and the estimated legal foreign-born population is an estimate of unauthorized immigrants included in the survey. Variants of the residual method have been widely used and are generally accepted as the best current estimates. 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 and for 35 countries or groups of countries by period of arrival in the United States.

The next 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 of the unauthorized immigrant population by 10-15%. For 2008, these procedures yielded an estimate of 11.9 million unauthorized immigrants in the United States, with 7 million from Mexico (Passel and Cohn 2008).

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 U.S. before 1980 are assumed to be legal immigrants. 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 (such as 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. As result of these steps, the foreign-born population is divided between individuals with “definitely legal” status (including long-term residents, naturalized citizens, refugees and asylees, legal temporary migrants, and some legal permanent residents) and a group of “potentially unauthorized” migrants.

The number of potentially unauthorized migrants exceeds the estimated number of unauthorized migrants (from the residual estimates) by 20-35%. So, to have a result consistent with the residual estimate of legal and unauthorized immigrants, probabilistic methods are employed to assign legal or unauthorized status to these potentially unauthorized individuals. This last step also involves a check to ensure that the legal statuses of family members are consistent; for example, all family members entering the country at the same time are assumed to have the same legal status. The entire process requires several iterations to produce estimates that agree 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 the foreign born 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.

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 and totals by country or region of birth. These estimates were presented in Passel and Cohn (2008).

CPS Weights

Official population estimates from the Census Bureau provide the basis for weighting the Current Population Survey and developing the population figures used in this report. In both 2008 and 2009, the Census Bureau introduced significant revisions in its population estimation methods. Because of these revisions, some potentially large differences over time in both numbers and characteristics of unauthorized immigrants could be due entirely to methodological changes and not to real changes in the population. Pew Hispanic Center estimates published in Passel and Cohn (2009) for 2003-2008 use revised population figures for 2003-2007 to produce consistent trend data. The figures shown here from the March 2009 CPS are not consistent with these previously published estimates. A new set of revised estimates for 2000-2008 will be released in a forthcoming report.

U.S.-Born, Legal Immigrant and Unauthorized Immigrant Households

A “U.S.-born household” is one where the head of the household and the partner or spouse (if present) are U.S. natives.

An “unauthorized immigrant household” is one where either the householder or spouse/partner (if present) is unauthorized.

In “legal immigrant households” the householder or spouse/partner (if present) is a legal immigrant. Neither the head of household nor the spouse/partner may be an unauthorized immigrant. Couples that consist of a U.S. native and a legal immigrant are classified as legal immigrant households.

A household that is defined as being a “couple with children” includes at a minimum a householder, a spouse or unmarried partner, and a minor child of either the householder or spouse/partner.