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.

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