Illegal Aliens

Despite Data Limitations, Current Methods Provide Better Population Estimates Gao ID: PEMD-93-25 August 5, 1993

During the 1980s, the methods used to measure the size and flow of illegal aliens were refined to yield a narrower range of estimates than was the case in a 1982 GAO evaluation (GAO/IPE-82-9). As a result, GAO projects the size of the U.S. illegal alien population during 1990 at no more than 3.4 million, a figure considerably below the 5.5 million maximum that the Census Bureau projected. Estimates of the gross annual flow of illegal aliens are more sketchy. For example, according to Immigration and Naturalization Service figures, the total inflow in 1988 was put at 2.3 million people. Owing to a high potential error in this calculation, however, GAO believes that the actual inflow could have ranged anywhere from 1.3 million to 3.9 million people. The main reason that this inflow does not translate into a larger illegal alien population is that much of it represents short-term visits to the United States.

GAO found that: (1) in the last decade, the range of estimates of the illegal alien population has narrowed; (2) sources for population size estimates include Census Bureau and Immigration and Naturalization Service (INS) population count data, death registration data, and Mexican census and sex ratio data; (3) the lack of information on legal status, geographic distribution, uncounted aliens, birth and death rates, and survey inconsistencies contribute to data limitations affecting population size estimates; and (4) sources for population flow estimates include the INS Nonimmigrant Information System, which collects airline-reported data on admissions and departures, and Border Patrol data on apprehensions of those caught attempting entry, but those sources collect information on two distinctly different alien populations. GAO believes that: (1) based on Census Bureau and 1990 Mexican census data, there were probably no more than 3.4 million illegal aliens resident in the United States in 1990, as compared to the maximum Census Bureau estimate of 5.5 million; and (2) it is more difficult to estimate the flow of illegal aliens, which is affected by how long illegal aliens remain in the United States and Border Patrol resources and productivity.

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