October 24, 1993
San Jose Mercury News
By Timothy Taylor
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"THE LIKELY maximum" number of illegal immigrants in
the United States is 3.4 million, according to the General Accounting
Office. But how can it know? How can anyone know?
Precise counts are impossible. But by comparing the results derived
from a variety of different statistical methods, one can state an
answer with reasonable confidence. The GAO report, called "Illegal
Aliens: Despite Data Limitations, Current Methods Provide Better
Population Estimates," describes five different methods for
estimating the number of illegal immigrants. The Residual Method.
Using data from the Census Bureau and the Current Population Survey,
calculate the total number of people who admit to being "foreign-born."
Then, using immigration statistics, calculate how many legal residents
are foreign-born. The gap between the two is one estimate of the
number of illegal immigrants.
The Death Registration Method. Using data on deaths in the United
States, separate out those who are foreign-born. Again, compare
the number with what would be expected, based on the available immigration
data, and use the difference to estimate the overall population.
A similar method is to analyze data on birth rates to find the number
of women of child-bearing age illegally in the United States.
The Sex Ratio Method. This method has focused on Mexican data,
since Mexico is believed to be the source of half to two-thirds
of illegal immigration. Examine the historical Mexican data on number
of births, and ratio of males to females. Based on that data, the
current Mexican census data show fewer Mexican men in the 15-39
age bracket than would have been expected. If one assumes that some
proportion of the missing men ended up in the United States, and
makes some assumptions about the characteristics of illegal immigrants
from Mexico -- for example, one might assume that they are 65 percent
male and 60 percent in the 15-39 age bracket -- one can use the number of missing men to project the overall inflow of illegal
immigrants from Mexico.
Non-Immigrant Overstay Method. The U.S. Immigration and Naturalization
Service records arrivals and departures for all those tourists,
business travelers, students and others who arrive with temporary
visas. In 1988, for example, there were 14.3 million arrivals. By
year's end, historical data would have predicted 13.7 million departures,
but only 12.3 million departures recorded. Some of the problem here
is that departure records aren't kept accurately; however, some
people with temporary visas are also becoming illegal immigrants.
Repeated Trials Method. Collect data from the U.S. Border Patrol
data on the number of people who were caught more than once trying
to enter the country, as identified by personal recognition of experienced
agents. Assume that anyone who tries repeatedly will get through
eventually, so the number of those caught more than once gives a
rough estimate of the flow of illegal immigration into the United
Obviously, each of these methods has its warts and flaws, discussed
in some detail by the GAO. But taken as a whole, it's the best the
government statisticians can achieve.
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