USING INS BORDER APPREHENSION DATA TO MEASURE THE FLOW OF UNDOCUMENTED MIGRANTS CROSSING THE US-MEXICO FRONTIER

This article examines how data on INS border apprehensions are related to the now of undocumented migrants crossing the southern U.S. border . Its centerpiece is a demographic model of the process of unauthorize d migration across the Mexico-U.S. frontier. This model is both a conc eptual framework that allows us to see theoretical linkages between ap prehensions and illegal migrant flows, and a methodological device tha t yields estimates of the gross number of undocumented migrants. One i mplication of the model is that, for the first time, the relation betw een apprehensions and illegal flows can be examined empirically. We sh ow that the ratio in each period between apprehensions and the undocum ented now is simply the odds of being located and arrested on any give n attempt to enter the United States clandestinely. In addition, data for 1977-1988 suggest that the simple linear correlation between the n umber of apprehensions and the volume of illegal immigration is approx imately 0.90 and that the size of the illegal migrant now is roughly 2 .2 times the number of Border Patrol arrests. The article concludes wi th a discussion of the conditions under which it is appropriate to use INS apprehensions data as an indicator for the flow of undocumented U .S. migrants.