May 1, 2013

A Demographic Portrait of Mexican-Origin Hispanics in the United States

Appendix B: Methodology

American Community Survey (ACS): 2011

The American Community Survey (ACS) is a continuously fielded survey that collects detailed information from a sample of the U.S. population on a wide range of social and demographic topics. Each month the ACS samples about 250,000 households. Interviews are conducted by mail and in person; follow-up interviews are conducted on a sample of initially non-responding households. The nominal sample size of the ACS is about 3.1 million households per year; about 2.1 million households are included in the final sample.6 The monthly samples do not overlap within five-year periods so that detailed information can be obtained for various geographic levels by combining samples across months.

Data from the ACS are released on an annual basis covering interviews conducted during calendar years. Information from a single year of ACS interviews is published for the nation, states, and “recognized legal, administrative, or statistical areas” with populations of 65,000 or more. Data for three consecutive calendar years are combined to provide tabulations for areas with populations of less than 20,000; data for five consecutive years provide information for all areas down to census tracts and block groups. The ACS began in 2005 with a sample of the household population and was expanded to full operational status in 2006 when the household and group quarters populations were included.

The ACS includes questions on place of birth (state or country), citizenship and residence one year before the interview. For people born outside the U.S., the ACS asks when the person came to live in the United States. These data items provide information on the foreign-born population in the U.S. and their movement to the U.S. ACS data presented in this report come from tabulations of microdata obtained from the Integrated Public-Use Microdata Series of the University of Minnesota (IPUMS). For each year, the microdata set represents a 1% sample of the U.S. population or about 3 million individual cases per year.

Like any survey, estimates from the ACS are subject to sampling error and (potentially) measurement error. Information on the ACS sampling strategy and associated error is available at http://www.census.gov/acs/www/methodology/methodology_main/. An example of measurement error is that citizenship rates for the foreign born are estimated to be overstated in the decennial census and other official surveys, such as the ACS (see Jeffrey Passel. “Growing Share of Immigrants Choosing Naturalization,” Pew Hispanic Center, Washington, D.C. (March 28, 2007)). Finally, estimates from the ACS may differ from the decennial census or other Census Bureau surveys due to differences in methodology and data collection procedures7

Decennial Censuses: 1850 through 2000

U.S. decennial censuses from 1850 through 2000 have provided information on the foreign-born population via a question on place of birth. Through 1970, these censuses also asked mother’s country of birth and father’s country of birth, which permit identification of the second generation. Data on the Mexican-born population from 1850 through 1990 are from these census results presented by Gibson and Jung (2006) and in the Historical Statistics of the United States (U.S. Census Bureau, 1975). Data for 1980 through 2000 come from the 5% public-use sample of census records from Integrated Public Use Microdata Series (IPUMS) to generate information on the foreign-born population. These sources also collect information on citizenship and year of entry to the U.S.

Data on the Mexican-origin population from 1850 through 2000 are based on Pew Hispanic Center tabulations of census records from the IPUMS.

Current Population Survey (CPS): 1995-2012

Monthly CPS

The Current Population Survey (CPS) is a monthly survey conducted by the Census Bureau for the Bureau of Labor Statistics. The CPS is a stratified probability sample of about 60,000 households designed to provide state-specific information on employment and unemployment (http://www.census.gov/cps/methodology/). Generally, about 50,000-55,000 households are interviewed. The sample has overlapping rotation groups in which each household is interviewed in four consecutive months, is out of the sample for eight months, and then returns to the sample for four more consecutive months.

The monthly CPS has a range of questions focused on labor force participation, but also collects information on demographic characteristics, education and immigration through questions on country of birth, parents’ country of birth and citizenship (since 1994). The citizenship information identifies respondents as U.S. natives, U.S. citizens through naturalization and non-citizens (but with no further information on legal status). The latter two groups comprise the foreign-born population. For persons born outside the United States, the CPS asks when the individuals “came to live in the United States.” All CPS cases are included in public-use microdata files, available from a variety of sources.

March Annual Social and Economic Supplement (ASEC)

Each March, the basic CPS sample and questionnaire are expanded for the Annual Social and Economic Supplement (ASEC). The sample is augmented to about 80,000 households with a double sample of Hispanic households and oversampling of households with children and households headed by persons who are not white. The questionnaire is expanded to include questions about health insurance, detailed sources of income, program participation and residence the previous March. This makes the March ASEC supplement the main source of information on poverty and lack of health insurance. The question on residence one year prior to the survey date provides information on current migration into the United States. In this report, the March CPS is the principal data source on the size and characteristics of the unauthorized population (see below for estimation methodology).

The published information from the CPS and the CPS microdata use survey weights based on the most current information available to the Census Bureau at the time the survey is conducted. Because additional data on population change can become available and because of changes in the methods used to measure population change, the weights for the monthly CPS and the March supplements are not necessarily consistent across time. Consequently, comparisons of population numbers across different releases of the CPS can conflate actual population change with methodological changes. To minimize the impact of methodological change on comparisons across time, the Pew Hispanic Center has developed alternative weights for the March CPS supplements of 1995-2012 that use a consistent set of population estimates and permit more accurate comparisons over time. The methodology for developing the alternative weights is described below.

Method for Weighting CPS 1995-2012 Data

CPS population figures are based on the Census Bureau’s official population estimates of the civilian, non-institutional population for the nation and states through a weighting process that ensures 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. The population estimates used to weight each March CPS are based on the latest available figures at the time the survey weights are estimated. Previous CPS weights are not revised to take into account updated population estimates.

This weighting process produces the best estimates available at the time of the survey, but does not guarantee that a time series produced across multiple CPSs is consistent or accurate.  Significant discontinuities can be introduced when the Census Bureau changes it population estimation methods (as it did several times early in the 2000s and in 2007 and 2008) or when the entire estimates series is recalibrated to take into account the results of a new census (as in 2012 for the 2010 census and 2001 for the 2000 census).

The estimates shown for the Mexican-born and Mexican-origin populations in this report are derived from March CPS files for 1995-2011 that have been reweighted to take into account population estimates consistent with the 1990 census, the 2000 census, the 2010 census and the 2012 population estimates. The population estimates used to reweight the March 2011 CPS come from the Census Bureau’s “Vintage 2011” population estimates (http://www.census.gov/popest/data/index.html); they are consistent with the 2010 census and the estimates used to weigh the March 2012 CPS. The population estimates used to reweight the March 2001 through March 2010 CPSs are the Census Bureau’s intercensal population estimates for the 2000s (http://www.census.gov/popest/data/intercensal/index.html); these population estimates use demographic components of population change for 2000-2010 and are consistent with both the 2000 and 2010 censuses. Similarly, the population estimates used to reweight the March 1995 through March 2000 CPSs are the intercensal population estimates for the 1990s (http://www.census.gov/popest/data/intercensal/index.html), which are consistent with the 1990 and 2000 censuses.

The reweighting methodology follows, to the extent possible, the methods used by the Census Bureau in adjusting the sample weights to the population totals. A more detailed discussion of the methods can be found in the Methodological Appendix to Passel and Cohn, 2010 (http://www.pewhispanic.org/2010/09/01/us-unauthorized-immigration-flows-are-down-sharply-since-mid-decade/) and in the Census Bureau’s documentation of CPS weighting procedures (http://www.census.gov/prod/2006pubs/tp-66.pdf).

Residual Method for Estimating Unauthorized Immigrant Population

The data presented in this report on unauthorized and legal immigrants from Mexico were developed with essentially the same methods used in previous Pew Hispanic Center reports (Passel and Cohn 2010; Passel and Cohn, 2009). The national and state estimates use a multistage estimation process, principally using March Supplements to the Current Population Survey (CPS).

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 (2011, 2010, 2008) and 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. The data and methods for the overall process were developed initially at the Urban Institute by Passel and Clark (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% to 3%, and the unauthorized immigrant population by 10% to 15%. The individual survey weights are adjusted to account for immigrants missing from the survey.