nationals account for the vast majority (around 90 percent) of attempted unauthorized crossings across the U.S.–Mexico border.

A discussion of the use of surveys and statistical methods to measure unauthorized border flows needs to be set in the context of border enforcement and the migration process, both current and past. The migration process is highly complex, and it is influenced by a variety of factors such as the economic, social, and environmental conditions in sending and destination areas; immigration policies; and interior and border enforcement. Furthermore, migrants and their smugglers have adapted, and can continue to adapt, to changes in resources and strategies on the U.S. side of the border, and enforcement efforts in one geographic area can have spillover effects into another. This situation argues for a broader conception of the border, not segmented into ports of entry and areas between the ports of entry. The migration process is also dynamic and evolving, and survey designs and modeling approaches that may be well suited for capturing certain aspects of unauthorized migration flows today may not be able to do so with the same reliability in the future. Thus, flexibility of design and continuous evaluation of how DHS is implementing border metrics will be of the utmost relevance.

Within this context, one can assess the usefulness of different types of data to capture information on migration flows. There are a number of major surveys in the United States and Mexico that collect some information about migration and border crossing. On the U.S. side, the American Community Survey (ACS) and Current Population Survey (CPS) each target U.S. households. On the Mexican side, the “long questionnaire” of the Mexican Census of Housing and Population (administered to a 10 percent sample of the population in Mexico), the National Survey of Occupation and Employment (ENOE), the National Survey of Population Dynamics (ENADID), and the longitudinal Mexican Family Life Survey (MxFLS) target households at a national level. None of these surveys was specifically designed to study migration. The Mexican Migration Project (MMP) and Mexican Migration Field Research Program (MMFRP), in contrast, focus on studying migration, although the surveys are not based on probability sampling. The Survey of Migration at the Northern Border (EMIF-N), which has a probability sample conceptual basis, targets migrants passing through northern border cities of Mexico.

The panel began by enumerating the major attributes for evaluating surveys for this purpose: the nature of the target population and related issues of sample size and survey design, the frequency with which surveys are conducted and the speed with which data are made publicly available, and the types of questions that are asked about migration. Since international migration is a relatively rare event, it is important for a general survey to have a sufficiently large sample in order to obtain reliable information on



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