Numbers, Facts and Trends Shaping Your World

More Mexicans Leaving Than Coming to the U.S.

Appendix B: Methodology

This appendix describes the specific items used from these various sources and the development of some of the migration measures used in the report. Data from the Mexican Survey of Demographic Dynamics (ENADID), the American Community Survey, the U.S. censuses of 1990 and 2000, and the Current Population Survey are based on Pew Research Center tabulations from public-use microdata sets, some of which have been modified or adjusted to better track changes in the U.S. Census Bureau’s population estimates, which may affect the trends seen in the data, and also to improve the coverage of recent immigrants in some of these instruments. As such, the figures reported may differ from published data from the same sources. The estimates presented in this report of immigrant flows between Mexico and the U.S. were rounded to the nearest 10,000, other figures included in this report were rounded to the nearest 1,000.

Mexican Data Sources

National Survey of Demographic Dynamics (Encuesta Nacional de la Dinámica Demográfica—ENADID): 2014

The National Survey of Demographic Dynamics or ENADID is a national household survey conducted by the Mexican government to collect a wide range of information about population change in Mexico. In addition to a module of questions related to international migration, the survey covers fertility and pregnancy history of women in detail, births and deaths, contraceptive usage and preferences, and marriage. The survey was conducted in 1992, 1997, 2009 and 2014 by Mexico’s National Institute of Statistics and Geography (INEGI) and by the National Population Council (CONAPO) in 2006. This report uses data from the 2014 collection only. Estimates for 2000 and 2010 migration flow from the U.S. to Mexico are based on a prior publication, for further detail on that report’s methodology, please see Passel et al., 2012.

Immigrants to Mexico and Mexicans returning from abroad are identified through a number of different questions. The main questionnaire asks respondents their state or country of birth, which is used to measure lifetime migration (identifying people born outside of Mexico). The main questionnaire also includes two questions on residence five years before the survey (respondents are asked about their place of residence in August 2009 – the state or country of residence and the municipality of residence if respondents were in Mexico). This is asked of people ages 5 or older. These two questions can be used to measure return migration of Mexicans during the five-year period before the survey or immigration to Mexico during the period by people not born in Mexico. The survey also provides data on migration patterns within Mexico. Since the question on residence five years prior pertains only to people ages 5 and older, the number of immigrants that came into Mexico among people younger than 5 years old is determined from the place of birth question, i.e., the number under age 5 born outside of Mexico.

The supplemental migrant questionnaire module is focused on international migration. The first question asks whether anyone who “lives or lived with you (in this house) went to live or work in another country” in the previous five years. People identified as leaving the country with this question are designated as “recent migrants” in this report. The ENADID also asks how many recent migrants left from the household and then follows with a battery of questions about each migrant: gender, age at most recent migration, month and year of most recent departure from Mexico, state of residence at departure, reason for departure, destination country and place of current residence (for example: U.S., other country, Mexico). For those migrants who are identified as currently living in Mexico, there are further questions: month and year of return, reason for return, and whether the returned recent migrant is in the respondent’s household. With these questions, it is possible to identify all people who migrated into Mexico during the previous five years who were still alive at the date of the survey (and were still in Mexico).

Total migration into Mexico from the United States during the five years prior to the 2014 ENADID’s administration is estimated from the main migration questions and the migrant sample of returned recent migrants.

Migrants from the U.S. to Mexico during 2009-2014 can be subdivided as:

  • People (ages 5 and older) living in the U.S. in 2009 and who were back in Mexico in 2014
  • People under age 5 in the survey who were born in the U.S.
  • Recent migrants who left Mexico after 2009 for the U.S. and returned by the survey date (excluding those living in the U.S. in 2009 and children under age 5 born in the U.S.)

Limitations of the ENADID

The ENADID has some limitations when it comes to providing full coverage of migration flows between Mexico and the U.S. All migrants into Mexico in the period before the survey (who are still alive and still in Mexico) can be identified. However, for recent out-migrants, only those migrants from households where some members remained in Mexico can be identified. ENADID is not able to measure outmigration of whole households.

The migrant sample includes only a limited amount of socio-demographic data on the migrants. However, most of the recent migrants who have returned to Mexico (i.e., those who returned to the same household) can be linked to their own record in the household and sociodemographic data. For 2014, we were able to match 90% the returned recent migrants (739 unweighted cases out of 818 returnees in the migrant sample). Because some migrants make multiple trips to the U.S., some of the returned recent migrants (i.e., those who made a trip out of Mexico after August 2009 in 2014) were living in the U.S. five years before the survey. In measuring total migration into Mexico, it is necessary to remove this group from the estimate to avoid double counting. Using the matched samples, 30% of the returned recent migrants in the 2014 ENADID had been in the U.S. five years earlier.

The data from ENADID employed in this report were developed from tabulations of microdata samples. The microdata come from a 93% sample of the full ENADID sample (101,000 households); all cases in the recent migrant sample are included in the microdata. For 2014, microdata samples were downloaded from the INEGI website (entered at especiales/enadid/enadid2014/default.aspx). The sample sizes for the microdata are: 94,422 households, 348,450 people living in those households and a sample of 2,289 recent migrants.

United States Data Sources

Current Population Survey (CPS): 2000-2014 March Annual Social and Economic Supplement (ASEC)

The Current Population Survey (CPS) is a national household survey conducted by the U.S. Census Bureau and the U.S. Bureau of Labor Statistics to collect a wide range of information about population in the United States. Each March, the basic CPS sample and questionnaire are expanded for the Annual Social and Economic Supplement (ASEC). The sample is increased to about 80,000 households with a double sample of Hispanic households and oversampling of households with children and households headed by people who are not white. The questionnaire also covers a much broader range of topics with questions about health insurance, detailed sources of income, program participation and residence as of 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 ACS is the principal data source on the size and characteristics of the unauthorized population for the years 2005-2012, while the March CPS is the main source for the years 2000-2004 and 2013-2014 (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 Research Center has developed alternative weights for the March CPS supplements of 1995-2011 that use a consistent set of population estimates and permit more accurate comparisons over time. The March 2000 CPS was reweighted to the 2000 Census; the 2003 to the intercensal estimates; and the 1995 and 1998 to 1990-2000 intercensal estimates. The methodology for developing the alternative weights is described in Appendix C of Passel and Cohn (2014).

American Community Survey (ACS): 2005-2013

The American Community Survey (ACS) is a continuous 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 is conducted on a sample of initially non-responding households. The nominal sample size is about 3.1 million households per year; about 2.1 million households are included in the final sample. 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.5

Data from the ACS are released on an annual basis covering interviews conducted during calendar years. 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 population 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 and movement to the United States. To the extent that ACS data are used in this report, the information comes from tabulations of microdata, released by the Census Bureau but with additional processing by the Integrated Public-Use Microdata Series (IPUMS). For each year, the microdata set represents a 1% sample of the U.S. population or about 3 million individual cases per year.

Each year’s ACS is weighted to the Census Bureau’s population estimates for that year. The 2010 ACS is the first to be weighted to results from the 2010 census. The use of annual population estimates for weighting can create discontinuities in making comparisons across years when the estimation methods change or when the results of a new census are introduced (as in 2010). To help minimize comparison issues related to changes in population estimates, the Pew Research Center has produced alternative ACS weights for 2005-2009 that are consistent with results from the 2010 census and the 2000 census (Passel and Cohn, 2012). These alternative weights are used in ACS results for 2005-2009. Note that estimates of the size of the foreign-born population from the ACS differ from those based on the CPS for a number of reasons. The surveys differ in weighting and coverage; the CPS universe is of the civilian, noninstitutional population while the ACS universe is of the total resident population. Additionally, our estimates from the March CPS and ACS are adjusted for survey undercoverage, and the 2005 ACS is augmented to include the group quarters population.

Decennial Censuses: 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).

For 1980 through 2000, we used a 5% public-use sample of census records from IPUMS to generate information on the foreign-born population. These sources also collect information on citizenship and year of entry to the U.S.

Estimation Methods

Two principal sets of estimates presented in this report were generated by the Pew Research Center using U.S. data sources described above and demographic estimation methods—estimates of the size and characteristics of the unauthorized immigrant population in the U.S. and estimates of the five-year inflow of Mexican immigrants into the U.S. for 2009-2014. Previous versions of these estimates and the methods used to derive them have been published elsewhere; see, for example, Passel and Cohn (2014), Passel (2007), Passel, Cohn and Gonzalez-Barrera (2012). This section includes a brief description of the estimation methods used in this report.

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 Research Center reports (Passel and Cohn, 2014; 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) and American Community Surveys (ACS).

The first stage in the estimation process uses CPS and ACS 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 and ACS; the difference is assumed to be the number of unauthorized immigrants in the CPS and ACS. 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 (2014, 2011, 2010, 2008) and Passel (2007) for more details.

Then, having estimated the number of legal and unauthorized immigrants in the March CPS Supplements and the American Community Surveys, 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 Research 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.

The estimates for 1995-2012 use specially developed survey weights for the CPS and ACS to ensure consistency across the years in the underlying population figures. (See Passel and Cohn 2010, 2014 for a detailed discussion of the need for these weights and their development.)

Five-Year In-Flows of Immigrants from Mexico

Detailed, accurate estimates of flows back and forth across the U.S.-Mexico border have been difficult to develop, particularly since the flow has been largely unauthorized. Census and survey data from the United States provide measures of the Mexican-born population in the United States and of flows of Mexicans who are living in the U.S. on a more or less permanent basis. The volume of temporary, seasonal or circular migration is harder to assess accurately.

Total immigration during any interval can be estimated from a demographic identity—change in the foreign-born population equals immigration less foreign-born emigration and deaths:

FBt+n – FBt = It,t+n – Dt,t+n – Et,t+n

Or, immigration equals foreign-born population change plus deaths and emigration:

It,t+n = (FBt+n – FBt) + Dt,t+n + Et,t+n

These boundary conditions provide the framework for measuring flows into the U.S. from Mexico. Survey-based estimates of all elements are subject to various measurement issues, including undercount, definitional inconsistencies, sampling and other errors. Developing consistent measures of annual immigration involves coping with these problems.

Estimates were developed for 2009-2014 using a combination of data sources, assumptions and measurement techniques, depending on the nature of the available information.

Estimated Flows, 2009–2014

The total number of arrivals for 2009–2014 is estimated from arrivals for 2012–2014 from the 2014 March CPS plus 2009-2011 arrivals from the 2012 ACS. These estimates were adjusted for undercount of recent immigration and differences in weights before and after the 2010 decennial census.

About the Pew Research Center’s Spring 2015 Global Attitudes Survey in Mexico

Results for the survey are based on face-to-face interviews conducted under the direction of Princeton Survey Research Associates International. The results are based on national samples, unless otherwise noted. More details about our international survey methodology and country-specific sample designs are available on our website.

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  1. For more information on ACS sample sizes and methods, see
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