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

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