The U.S. Department of Homeland Security (DHS) is responsible for securing and managing the nation’s borders. Over the past decade, DHS has dramatically stepped up its enforcement efforts at the U.S.–Mexico border, increasing the number of U.S. Border Patrol (USBP) agents, expanding the deployment of technological assets, and implementing a variety of “consequence programs” intended to deter illegal immigration. During this same period, there has also been a sharp decline in the number of unauthorized migrants apprehended at the border.
Trends in total apprehensions do not, however, by themselves speak to the effectiveness of DHS’s investments in immigration enforcement. In particular, to evaluate whether heightened enforcement efforts have contributed to reducing the flow of undocumented migrants, it is critical to estimate the number of border-crossing attempts during the same period for which apprehensions data are available. With these issues in mind, DHS charged the National Research Council (NRC) with providing guidance on the use of surveys and other methodologies to estimate the number of unauthorized crossings at the U.S.–Mexico border, preferably by geographic region and on a quarterly basis. The NRC appointed the Panel on Survey Options for Estimating the Flow of Unauthorized Crossings at the U.S.–Mexican Border to carry out this task. A better understanding of the magnitude, timing, and location of these flows will help DHS to better evaluate the effectiveness of its enforcement efforts, more efficiently allocate its resources along the border, and provide a more complete report to the public on the state of illegal immigration. This study focuses on Mexican migrants since Mexican
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
migration. In particular, the sample size requirements for traditional national household surveys would be considerable, especially if one wanted precise flow estimates by the nine geographic sectors into which USBP divides the southwest border. The ever-changing nature of migration flows would also create design challenges for geographically specific estimates. Although sample sizes can be smaller for probability surveys that focus on migrants (such as EMIF-N), changing patterns of migration would still create design and analysis challenges. Moreover, although EMIF-N shows great potential for DHS purposes, its adaptive design makes assessing its accuracy problematic.
Estimating annual flows in a timely fashion using survey data is a great challenge, and doing so on a quarterly and border sector/subregion basis is an even greater challenge. For border flow estimates to have practical value, survey data would need to be collected, analyzed, and released in a timely fashion. Most of the existing surveys do not meet these criteria, and only one—ENOE—is currently capable of providing quarterly estimates on a timely basis (although EMIF-N might also have the potential to provide such timely data). There are a number of key bits of information on migration and border crossing that ENOE does not collect (e.g., documentation status at crossing and border crossing location); questions on those items could, in principle, be added to the ENOE survey instrument. Given the dynamic nature of the migration process, however, questions about the migration process that are salient today may be less so in the near future. Thus, the questions added would have to be limited.
Such improvements could prove useful to researchers and others, and they would be welcome by the panel (as would, for example, improvements in the timeliness of EMIF-N). But from the perspective of estimating flows on an annual or quarterly basis, such modifications would still take place against the backdrop of larger limitations and complexities relating to sample size and survey design. The report also notes the administrative and implementation challenges arising from the fact that ENOE falls under the jurisdiction of the government of Mexico. These challenges would be no less salient if the U.S. government attempted to put in place a new dedicated migration survey in Mexico. Although surveys, especially those that focus specifically on migration, can provide a wealth of information about the migration process, they are not sufficient by themselves in meeting the needs of DHS for obtaining estimates of unauthorized migration flows across the U.S.–Mexico border on an annual or quarterly basis. The panel believes that, although DHS could benefit from engaging with entities in Mexico that collect survey data relevant to the analysis of unauthorized migration, it should not invest substantial resources in making changes to existing surveys or in implementing a new survey for the purpose that is the subject of this study.
Apprehensions data collected by USBP in its enforcement database are also insufficient by themselves to estimate flows because those data do not contain information on unauthorized migrants who elude capture or on migrants who are deterred from trying to cross again after they were apprehended once or more. Data on the re-apprehensions of individuals do provide information that could, in theory, be used to make inferences about the size of the unauthorized population entering the United States successfully. The accuracy of those inferences, however, depends on modeling assumptions that are often difficult to validate.
Despite the limitations of DHS administrative data, their combination and integration with survey data could produce useful insights about migrant flows and the effectiveness of border enforcement. The panel was charged with reviewing administrative data collected by DHS, and in order to better inform our analysis and discussion, we formally requested that DHS provide us with apprehensions data from the USBP enforcement database. The panel made its request with the understanding that any data given to us would need to be made publicly available, in accordance with the institutional rules governing NRC studies. We indicated that DHS could provide it to us in a format that would protect any information that it deemed operationally sensitive. However, DHS would not provide these data without an exemption from public disclosure requirements. It was the judgment of the panel that the quality of its published analysis and the timeliness of its deliberations would have been unduly impaired by the classification restrictions that would have accompanied such an exemption. Therefore, the panel did not pursue its request, and DHS did not provide the panel with access to its administrative data.
The panel had sufficient information about the content, populations covered, and other features of DHS administrative data to support its conclusions about the possible contributions of the data to estimating unauthorized crossings at the U.S.–Mexico border. However, the panel believes that researcher access to DHS data would improve the quality of research in this area and contribute to a better understanding of activity at the border. Researcher access would also permit independent scholars to evaluate the quality, completeness, and reliability of the DHS data at the micro level, which has never been done and which would permit a much richer understanding of the role that apprehensions data could play in informing model-based approaches to estimating flows. Moreover, knowledge of and experience with the use of model-based approaches for estimating flows have been limited to date, and the complexities and uncertainties are considerable. To develop, apply, and continually refine specific modeling approaches, DHS will need to engage with the broader scientific community in a sustained and long-term fashion. This will be possible only if the data in its enforcement databases are made widely available.
DHS might be reluctant to release apprehension and other administrative data to the public on the grounds that they are sensitive from a law enforcement point of view. The panel believes that most of these concerns can be addressed by implementing masking methods for problematic fields in the records, creating broad geographic identifiers that link them to USBP sectors rather than individual USBP stations, and releasing data with sufficient delay (e.g., a full year) to diminish their sensitivity for operational use and deployment.
Given the importance of looking at the border as an integrated whole, the panel also believes that DHS should integrate apprehensions data for analytical purposes from the Office of Field Operations (OFO), which is responsible for enforcement at official ports of entry; from USBP, which is responsible for enforcement between ports of entry; and from Immigrations and Customs Enforcement (ICE), which is responsible for immigration violations in the interior of the country, away from the physical border itself. Conversations with representatives from DHS suggest that that the linkages between the apprehensions records controlled by USBP, OFO, and ICE in the ENFORCE database are limited to uses that relate specifically to enforcement; linkages across the data sources for broader analytical purposes would require approval from each of the three agencies, and the full database has not been widely used for analysis. Sharing of data across the different components of DHS for analytical purposes would seem a reasonable starting point.
Given the gaps and limitations in survey and administrative data, the panel believes that a necessary approach to estimating the flow of undocumented migrants consists in developing models that can combine survey, administrative, and other types of disaggregated data. These modeling approaches could include conventional statistical regression and other models and incorporate spatiotemporal aspects of the data, but they might also include less-standard simulation-based approaches such as agent-based models.
Any modeling approach and the assumptions underlying it will need to be continually validated against historical trends and data. Although each of these approaches has its limitations, much could also be learned by comparing estimates from these multiple methods. However, without access to DHS data, the panel is unable to provide precise guidance on the modeling approaches that would be most useful for estimating migration flows. Furthermore, although the panel was aware that DHS has been considering specific modeling approaches (e.g., capture-recapture methods using apprehension data), it was not granted access to the relevant technical reports. Moving forward, DHS would greatly benefit from making the administrative data in its enforcement databases (which could be subject
The major conclusions and recommendations offered by the panel (and numbered according to the chapter in which they are developed) are as follows:
• Conclusion 2.1: To understand migration flows in any one sector, it is important to view the entire border as a system; localized increases in border enforcement may simply change where migrants cross without reducing the overall flow in the long run.
• Conclusion 2.2: The migration process is complex and dynamic. Undocumented migration is the outcome of many interrelated factors that can vary widely across people, space, and time; migrant characteristics and the geography of sending and destination areas are changing constantly. This complexity and dynamism should be incorporated into the analytical approaches and study designs used to estimate flows of unauthorized migrants.
• Recommendation 4.1: For the purpose of estimating unauthorized migration flows across the U.S.–Mexico border on an annual or quarterly basis, DHS should not invest substantial resources in making major changes to existing surveys or in implementing a new survey.
• Conclusion 4.1: Existing surveys are subject to a variety of limitations having to do with target populations and associated 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. Therefore, although survey data are critical for understanding patterns and general trends in unauthorized migration, they will not be sufficient by themselves to meet the needs of DHS for estimating unauthorized migration flows across the U.S.–Mexico border.
• Conclusion 4.2: Implementing a new household survey that meets the needs of DHS would require an investment at least comparable to that associated with the American Community Survey in the United States; any such survey would also have to fall within the purview of the Government of Mexico. A survey that uses a time-location design and focuses directly on migrant populations (e.g., EMIF-N) would be more promising, but such a non-traditional
design would necessitate careful adherence to the sampling protocol and, in particular, would require that concerns about coverage error be addressed. Mexican-side implementation would also be an issue. Substantial modifications of existing general household or specialty migration surveys to meet the needs of DHS would encounter similar challenges. These challenges are only magnified by the complex and dynamic nature of the underlying migration process.
• Recommendation 5.1: DHS should integrate apprehensions data from USBP, OFO, and ICE for analytical purposes.
• Conclusion 5.1: Administrative data from DHS are alone insufficient to estimate the flow of unauthorized migrants across the U.S.–Mexico border. However, they could be combined with survey data to produce useful insights about migrant flows and the effectiveness of border enforcement. The use of modeling approaches in conjunction with disaggregated survey and administrative data is necessary for estimating these flows.
• Recommendation 5-2: DHS should sponsor and conduct research on modeling approaches for estimating the flows of unauthorized migrants across the U.S.–Mexico border.
• Conclusion 5.2: DHS would greatly benefit from making the administrative data from its immigration enforcement databases publicly available for research use, as that would allow DHS to engage with the broader scientific community to develop, apply, and continually refine specific modeling approaches. DHS could develop ways of constructing masked and/or aggregate files for public release in order to protect sensitive information.
• Conclusion 6.1: Modeling approaches, and the assumptions underlying them, must keep track of changing mechanisms of migration and be continually validated against historical trends and data. Since all modeling approaches have their limitations, there is also much that could be learned by comparing estimates from multiple methods.