Appendix B: Methodology

The data presented in this report on unauthorized and legal immigrants were developed with essentially the same methods used for previous reports (Passel and Cohn 2010, 2009, 2008). The state-level estimates for 2010 and 2007 are based on a variant of previous methods (e.g., Passel and Cohn 2010, Pew Hispanic Center 2006). The national and state estimates use 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 by the Census Bureau for the U.S. Bureau of Labor Statistics; 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, which are 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. For more details, see Passel and Cohn 2010, 2008; and Passel 2007.

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. 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 and in previous reports.

All estimates shown for 2000-2009 are identical to those in Passel and Cohn 2010 and Passel and Taylor 2010. The estimates for 2000-2008 use specially developed survey weights for the CPS to ensure consistency across the years in the underlying population figures. (See Passel and Cohn 2010 for a detailed discussion of the need for these weights and about their development.)

State-level Estimates

State-level estimates should be treated with some caution because they are based on much smaller samples than the national estimates. Estimates from single years can be extremely volatile, so measurement of trends over time can be unreliable. To provide interpretable trends, previous estimates have relied on multiyear averages and regression methods (Passel and Cohn 2010, 2009; Pew Hispanic Center 2006).

The estimates presented here for states in 2010 are based on tabulations of the augmented March 2010 CPS file where the sample sizes exceed 50 unauthorized immigrant households (unweighted). There are 16 states and the District of Columbia where the single-year estimates are used for 2010: Arizona, California, Colorado, Connecticut, District of Columbia, Florida, Georgia, Illinois, Maryland, Nevada, New Jersey, New York, North Carolina, Oregon, Texas, Virginia and Washington. These states had more than 81% of the nation’s estimated unauthorized immigrants in 2010. For the 34 states with fewer than 50 unauthorized immigrant households, the estimates shown for 2010 are an average of 2009 and 2010 CPS-based estimates. These states are Alabama, Alaska, Arkansas, Delaware, Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Hampshire, New Mexico, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, West Virginia, Wisconsin and Wyoming. For these same states, the workforce estimates in 2010 also are based on 2009-2010 averages. The total number of unauthorized immigrants does not differ significantly for these two years and the distribution across states is more similar for 2009 and 2010 than for any pair of years between 2000 and 2010. The very high degree of similarity suggests that averaging to reduce sampling variability does not distort the trend analysis.

The state estimates for 2007 are derived by averaging the distributions of unauthorized immigrants across states. These average percentages were then applied to the national total for 2007 of 12.0 million unauthorized immigrants to derive the state estimates. The use of three years of data reduces substantially the margin of error of the resulting estimates. The distributions across states are quite similar for these years. The dissimilarity index for the 2006-2007 pair is smaller than all others except the 2009-2010 pair noted above. The index for 2007-2008 is the fifth smallest, behind 2009-2010, 2006-2007, 2000-2001 and 2001-2002. Margins of error for the state-level estimates are derived with replicate weights developed by the Census Bureau for the March Current Population Surveys of 2005-2010 (U.S. Census Bureau, 2010).

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.