Counting undocumented migrants, trafficking victims or not, involves statistical acrobatics

I continue to point out that all statistics for victims of trafficking are not only estimates but often irresponsible guesses. Workers in informal-sector jobs, whether they are migrants or not, and no matter what job they do and whether sex is involved or not, have not officially registered their presence as residents or workers, meaning there are no files or databases, to consult. For more on statistical acrobatics, particularly on sex trafficking, check out these articles.

Methods for estimating undocumented migrants do exist (undocumented migrants being the framework in which trafficking victims should be located). In the following example, the Pew Hispanic Center (in Washington DC) publishes its new figure (11.1 million in March 2009), asserting that their method of calculation, the residual method, is reliable and widely accepted because based on ‘official government data’. They explain that:

Under this methodology, a demographic estimate of the legal foreign-born population—naturalized citizens, legal permanent residents, temporary legal residents and refugees—is subtracted from the total foreign-born population. The remainder, or residual, is the source of population estimates and characteristics of unauthorized immigrants. These Pew Hispanic Center estimates use data mainly from the Current Population Survey (CPS), a monthly survey of about 55,000 households conducted jointly by the U.S. Bureau of Labor Statistics and the Census Bureau. It is best known as the source for monthly unemployment statistics. Each March, the CPS sample size and questionnaire are expanded to produce additional data on the foreign-born population and other topics. The Pew Hispanic Center estimates make adjustments to the government data to compensate for undercounting of some groups, and therefore its population totals differ somewhat from the ones the government uses. Estimates for any given year are based on a March reference date. From U.S. Unauthorized Immigration Flows Are Down Sharply Since Mid-Decade

The Pew says they use the Current Population Survey. That is a census exercise, in which a form is sent to households to fill out. Undocumented migrants have abundant reasons for not filling in census forms correctly (and there are no penalties for filling them in incorrectly). So undercounting is likely. The Pew Center know that and make an adjustment, but the range of adjustment methods is also very wide:

All known users of this methodology correct the foreign born population (about 35–50 million) by 10–40% (3–12 million) to account for this undercount effect. Critics claim this correction is in error no matter which size correction is used. Wikipedia

About the Pew estimate, the Migration Information Center, also in Washington, warns this finding is inconclusive because of the margin of error in the estimates, which are based on data from the Census Bureau’s Current Population Survey of 50,000 households each month. From a December 2010 report from the Migration Information Center (US migration).

In 2004, Time magazine claimed more dramatic numbers without feeling it necessary to explain how they were arrived at:

It’s fair to estimate, based on a Time investigation, that the number of illegal aliens flooding into the U.S. this year will total 3 million–enough to fill 22,000 Boeing 737-700 airliners, or 60 flights every day for a year. It will be the largest wave since 2001 and roughly triple the number of immigrants who will come to the U.S. by legal means. (No one knows how many illegals are living in the U.S., but estimates run as high as 15 million.)

I hope it’s clear that everyone’s guessing in one way or another!

Laura Agustín, the Naked Anthropologist

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