redirect attempts at entry to other sectors, implying that analysis of enforcement requires looking at the border as an integrated whole.

That said, administrative data from DHS are alone insufficient to estimate the flow of unauthorized migrants across the border. These data report the number of individuals who are captured by USBP at entry but contain no information on those who elude capture. Data on re-apprehensions of individuals provide insight into the migration process but do not carry information about the “got aways.” Because those apprehended may either try to cross the border again or return to their home in interior Mexico, knowing the fraction of those re-apprehended does not allow one to identify the probability of apprehension. Without knowledge of the apprehension probability, one cannot use the level of apprehensions to make inferences about the flow of undocumented migrants across the border. It is unlikely that having access to the DHS administrative data would have changed the panel’s conclusion.

Despite their limitations, the administrative data still have many uses for understanding unauthorized migration. By combining administrative data with survey data, one can produce a model of individuals’ migration decisions in Mexico that is informative about how attempts at illegal entry respond to changes in the economic environment. Such an approach would combine a behavioral model of the decision to migrate, analyzed using survey data, with a model of the stochastic process governing apprehensions, analyzed using administrative data. Although this approach can produce estimates of the flow of unauthorized migrants across the border, it incorporates assumptions about migrant behavior and the statistical properties of apprehensions that may not be open to empirical validation. Still, models of migration that combine administrative and survey data would provide DHS with additional tools for analyzing the effectiveness of border enforcement and expected future workloads for USBP agents. Chapter 6 examines in more detail a variety of modeling techniques and approaches.

Because DHS administrative data have not been made public, they have never been evaluated by independent scholars for their quality, completeness, or reliability—an omission that is significant in light of the role that apprehensions data could play in informing model-based approaches to estimating flows. As the discussion in Chapter 6 will also make clear, knowledge of and experience with the use of model-based approaches for estimating flows are so far limited, and the attendant complexities and uncertainties are considerable. In order 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 administrative data discussed in this chapter are made widely available.

Currently, however, DHS shares its administrative data on apprehensions of



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