Technical Appendix

This appendix presents detailed information on the following technical topics: (1) the estimation of “system error” (percentage of uncounted departures) and INS’ “global” assumption that the level of system error is the same worldwide (across all airlines and all countries of citizenship); (2) the formula we developed for estimating overstays (given an estimate of system error) and an explanation of the error we identified in INS’ computation formula; and (3) a new set of procedures for estimating overstays that, by incorporating data on specific airlines, avoids the need for INS’ global assumption and, at the same time, uses the correct computation formula and appropriately weighted data. Use English. It is clear, however, that being able to estimate the number of uncounted departures (system error) would allow the number of overstays to be estimated. INS Devised a Strategy to Estimate System Error

INS devised a creative strategy to estimate system error. The key is to examine the rates of unrecorded departure forms in “index” countries—such as Australia, Sweden, Finland, and Switzerland—for which INS believes it is safe to assume that there are only negligible numbers of overstays. Notably, Mexicans and Canadians who arrive by air may be more likely to depart by land than visitors from countries that do not border the United States. Such a variable could form the basis of a new index. XXV, No. U.S. Immigration and Naturalization Service, Statistical Yearbook of the Immigration and Naturalization Service, 1990.
Why GAO Did This Study

Pursuant to a congressional request, GAO examined the basis for the Immigration and Naturalization Service (INS) estimates of persons who entered the United States legally as visitors but did not leave under the terms of their admission.

What GAO Found

GAO found that INS: (1) devised a creative approach for estimating overstays by estimating the number of uncounted departures; (2) determined that the number of uncounted departures could be estimated by using system data from countries for which it seems safe to assume there are few or no overstays; (3) applied its strategy using a global approach that requires the assumption that a single rate of system error applies to all countries worldwide; and (4) global approach provided a good starting point for estimating overstays, but INS should develop improved procedures for estimating overstays among foreign visitors arriving by air.