other interested parties. We will also make copies available to others upon request.

If you or your staff would like to discuss any of the issues we present here, please call me at (202) 512-2900 or Judith A. Droitcour, who served as project director on this study, at (202) 512-5885. Other major contributors to this report are listed in appendix IV.

Sincerely yours,

Joseph 7. Delfind

Joseph F. Delfico

Acting Assistant Comptroller General

Appendix I

Technical Appendix

Background

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. Additional sections
of this appendix discuss (4) possible further improvements in estimation
procedures and (5) problems with overstay estimation procedures for
visitors arriving by land.

Each year, millions of foreigners legally enter the United States as
“nonimmigrant" visitors (that is, on a temporary basis and for a specific
purpose, such as tourism). The INS Nonimmigrant Information System
tracks records of legal nonimmigrant visitors by country of citizenship;
visa category (tourist visit, business, or other purpose such as temporary
work); mode of travel (air, land, or sea); and for each air passenger, the
airline on which he or she entered the United States.1 As indicated by the
NIIS data, the large majority of foreign visitors are tourists who enter by air.
(It is important to note that many Canadian visitors are not counted by this
system. The same is true for many Mexican visitors who use
border-crossing cards.2 Data on foreign students entering the United States
to attend school are maintained in a separate system and are not part of
the estimates discussed in this report.)

INS also attempts to determine whether or not each foreign visitor
(identified by name, date of birth, country of citizenship, and passport
number) exits the United States by 9 months after his or her expected date
of departure. INS' use of a minimum 9-month overstay period for purposes
of calculating the overstay estimates discussed in this report means that
transients who overstayed for only a few weeks or months are not
included in these figures. Rather, INS defines estimated overstays as

'The NIIS data system includes a record of information that visitors provide on arrival/departure forms, including sex, age, and so forth; port of entry is also recorded in the data system. We note that an unknown number of persons fraudulently enter this country as tourists. If the fraud is successful, the record of such a person's seemingly legitimate entry would be tracked together with legal entries in the NIIS system.

2For Mexicans with border-crossing cards, travel is limited to an area within 25 miles of the
U.S.-Mexican border for periods no longer than 72 hours.

Technical Appendix

foreign visitors who-having been here for a year or more-appear to have settled in the United States. However, as explained below, there is (1) considerable error in counting visitor departures and thus

(2) uncertainty in the estimation of overstays.

Briefly, when a foreign visitor legally enters the United States, he or she fills out an arrival/departure form (I-94 form; see figure I.1).4 The arrival portion of the I-94 form is detached and retained by INS. The departure portion of the form is stapled to the visitor's passport. Each visitor is supposed to turn in the departure portion of the form when exiting the United States. The INS data system attempts to match the arrival with the corresponding departure portion of the form. Thus, it is possible to determine the number of foreign visitors for whom departure forms have not been recorded ("unrecorded departure forms").

In some instances, unrecorded departure forms correspond to visitors who overstayed. Often, however, departure forms have gone unrecorded because either (1) the forms were not turned in, not collected, or if collected by an airline, not returned to INS; or conceivably, (2) the forms were returned to INS but not correctly recorded as of a 9-month period following the expected departure date.

3For example, a tourist with a 3-month visa who overstays for 9 months has been in the United States for a year. Note that the procedures described here estimate the number of new illegal immigrants who are overstays. Certain analyses transform these estimates to estimates of the number of illegal immigrants-living here as of any one date-who initially came in legally and overstayed. To perform this transformation, it is necessary to estimate the number of overstays who returned to the home country before the date in question, the number who adjust to a legal status before that time, and so forth.

"Visitors from countries in the Visa Waiver Pilot Program use form I-94W. The current Visa Waiver Pilot Program countries are Andorra, Austria, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, Netherlands, New Zealand, Norway, San Marino, Spain, Switzerland, Sweden, and United Kingdom. Effective April 1, 1995, Ireland was also designated as a pilot program country, but with probationary status.