[TheMercenary]

This is what I was speaking about.

These estimates and claims rest on several annual efforts to count illegal immigrants in the U.S. The nonpartisan Pew Hispanic Center estimated that in 2008 the nationwide population was 11.9 million, and half a million in Arizona. The federal Department of Homeland Security and the Center for Immigration Studies, a Washington, D.C., research group that opposes increased immigration, agree on a figure of 10.8 million for 2009, with DHS putting the Arizona population at 460,000, down from 560,000 a year earlier.

But as my print column notes this week, these estimates are limited by several factors that make it difficult for researchers to count this population. No major government survey, including the decennial census now under way, asks Americans about their citizenship status. Thus estimates of the number of illegal immigrants in the country are indirect and possibly far off from the correct count.

These studies rely on census surveys, and assume that about 10% of illegal immigrants aren’t counted in these surveys. But that figure largely is based on a 2001 survey of Mexican-born people living in Los Angeles. “I do not advise use of my estimated undercounts for the 2000 census outside of L.A. county, nor for migrants from other nations,” said study co-author Enrico Marcelli, assistant professor of sociology at San Diego State University. “However, demographers do not have any other empirical evidence at the moment with which to proceed.”

One concern is that the nearly two in five households who didn’t respond to the 2001 survey may have included a disproportionately large number who also didn’t respond to census interviewers. Marcelli said further study would be needed to test that possibility, but he noted the extent of the efforts to select a representative sample and to put respondents at ease in order to elicit honest answers.

“As far as I know, there has not been a new, serious attempt to estimate the undercount of illegal immigrants in the census,” said Steven Camarota, director of research for the Center for Immigration Studies.

Further...

Larger estimates also sometimes are based on border-patrol counts of apprehensions, which are far from reliable proxies. No one is sure of how many people are missed for each one who is caught trying to cross into the U.S. illegally. Many of those who do get through may return quickly, or cross back and forth. Also, some people are caught more than once, inflating the count.

“It seems like we’re not missing that many bodies in the United States,” said Camarota, referring to the gap between the 20 million figure and his own.

The immigrant counters generally have seen a decline in the illegal-immigration population. “Economic drivers are very, very powerful” in lowering the illegal-immigrant population, said Hans Johnson, associate director of the Public Policy Institute of California. Others point to stepped-up enforcement efforts.

However, because of all the assumptions baked into these numbers, such drops come with so much statistical uncertainty that they may not be statistically significant. “The methodology for doing these estimates is not really designed to measure year-to-year change,” Passel said.

http://blogs.wsj.com/numbersguy/the-...mmigrants-937/