Nevertheless, the Office of Immigration Statistics (OIS) at DHS did provide information on the main fields included in the apprehensions database. Non-DHS data sources such as the Survey of Migration at the Northern Border of Mexico (EMIF-N) also provide some information on the apprehension of migrants at the border. In this chapter, we use the descriptions of the DHS data sources provided to the panel by OIS, as well other non-DHS data, to assess the usefulness of administrative data for measuring the flow of unauthorized migrants into the United States.

Our conclusion is that administrative data alone do not permit reliable estimation of the inflow of unauthorized migrants across the U.S.–Mexico border. The data provide no direct information on the number individuals who elude capture and enter the United States successfully. By making assumptions about the behavior of unauthorized migrants, one can use the volume of apprehensions to estimate the magnitude of unauthorized flows (see, e.g., Espenshade, 1995b; Massey and Singer, 1995). In this chapter, the panel discusses how one can generalize an approach, previously developed and reported in the sociology literature, which uses a “repeated trials” model. We note that all such estimates are based on strong assumptions that are difficult to validate empirically. As discussed in this chapter, estimation methods based on capture-recapture techniques (described in detail in Appendix B), which offer a sophisticated approach to determining the size of a population based on the fraction of initially “sampled” (i.e., apprehended) individuals who are subsequently “re-sampled” (i.e., re-apprehended) cannot, unfortunately, solve the problem about the lack of direct information on the number individuals who elude capture and enter the United States successfully. While administrative data have limitations, they could still offer potential insights into unauthorized migration flows if they were combined with other data sources. The panel outlines some strategies for characterizing key features of unauthorized migration flows, based on combining administrative and survey data. These approaches are discussed in more in detail in Chapter 6.

SOURCES OF ADMINISTRATIVE DATA ON APPREHENSIONS

USBP seeks to apprehend all individuals who attempt to cross U.S. borders illegally. Data on these apprehensions are a major source of DHS administrative records on unauthorized migration. In the last decade, USBP resources have increased dramatically, with the number of USBP officers growing from 9,000 in 2001 to 21,000 today (see Kessler, 2011). The expansion in resources, combined with the drop-off in the number of migrants apprehended at the U.S.–Mexico border since 2007, means that USBP currently has the manpower to document virtually all individuals with whom it comes in contact. But it also creates difficulties for comparing information



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