Trends in total apprehensions are, by themselves, inadequate because different outcomes may be given similar interpretations (Morral et al., 2011). For example, increases in border apprehensions may be suggestive of more effective enforcement as long as the underlying flow of unauthorized immigrants is declining, constant, or increasing by a smaller amount than the rise in apprehensions. However, declines in border apprehensions might also be suggestive of more effective enforcement as long as the magnitude of the decline in apprehensions is larger than the magnitude of the decline in the underlying flow of unauthorized immigrants.4 In both instances, there is an increase in the ratio of apprehensions to unauthorized crossings.5

DHS has charged the National Research Council (NRC) with providing guidance on the use of survey options 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 (see Box 1-1). 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, allocate its resources along the border more efficiently, and provide a more complete report to the public on the state of illegal immigration.

Effective enforcement by DHS has at least three aspects: first, stopping individuals who are in the process of attempting to enter the United States from succeeding; second, doing this so well that potential entrants stop trying and return to their homes; and third, discouraging potential migrants from even leaving their homes in order to attempt illegal entry into the United States. The focus of most DHS measurement attempts, most surveys, and this report is on the first two of these outcomes. Full assessment of enforcement success, however, would require measuring the third outcome—discouragement at the origin. The measurement of this third outcome is beyond the scope of this report. Broad surveys of the Mexican population and complex analyses of the characteristics of potential migrants would be required to assess such discouragement.

Chapter 2 of the report provides context for the use of surveys and statistical methods for estimating flows by providing an overview of recent estimates on the stocks and flows of undocumented immigrants; a brief history of unauthorized immigration and policy responses to it, including a short summary of studies looking at the effectiveness of border enforcement; and a description of the border crossing process for unauthorized

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4 If enforcement efforts have a deterrent effect and cause the underlying flow of unauthorized immigrants to decline, enforcement can be regarded as effective even if the magnitude of the decline in apprehensions is smaller than the magnitude of the decline in-migration.

5 However, as suggested in the previous note, the ratio of apprehensions to border crossings may not be completely informative about the effectiveness of enforcement efforts if those efforts also have a deterrent effect on unauthorized immigration.



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