from other sources (e.g., the cost of day labor or the number of illegal aliens found during random traffic stops).

TELEPHONE CARDS: A THOUGHT EXPERIMENT ON QUANTIFYING A DIFFICULT-TO-MEASURE POPULATION

Two glaring gaps in the information required to estimate the effectiveness of the resources that have been deployed at the U.S.–Mexico border during the past decade are the proportion of undocumented crossers who succeed in their first or later attempts and the proportion of apprehended migrants who are deterred from further crossing attempts. In the course of its deliberations, the panel discussed a number of different ideas concerning creative sampling methods for estimating different components of undocumented immigration. The panel describes here one such idea for quantifying one type of deterrence effect (i.e., the fraction of apprehended migrants who choose not to attempt to cross the border again). This simple thought experiment involves providing telephone cards to undocumented immigrants who are apprehended in the United States and are then returned to the Mexican side of the border.

Typically, individuals who intend to cross the border without documentation arrive in the border area and make arrangements for illegal crossing with assistance from a smuggler. Most of those who are apprehended during their first attempt and are returned to the Mexican side of the border will try to cross again within the next few days. If the second attempt is also unsuccessful, they tend to keep trying, usually over a period of several days, until finally they either succeed or give up. USBP could, in principle, provide a phone card from a Mexican telephone company to a randomly selected subset of apprehended migrants who are about to be returned to Mexico. The phone cards, which would come preloaded with a certain usage value, could be used to call from either side of the border, but only after the caller is identified as the person who actually received the card. The toll-free number to activate the card would differ depending on whether the individual was in the United States or Mexico at the time of activation. The fraction of individuals activating the card in Mexico would provide an estimate of the fraction of apprehended individuals who are deterred from crossing again, and the fraction activating the card in the United States would provide an estimate of the fraction of individuals that cross successfully on the next attempt.

Several practical problems would need to be resolved in order to implement the phone card experiment. First, phone cards would have to be sufficiently attractive so that migrants actually use them, but not so valuable that they render the program too expensive (or induce criminal elements to prey on returning migrants, or create a black market in phone cards). This



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