By the numbers: How America tallies its 11.1 million undocumented immigrants

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Undocumented immigrant Oscar Rodriguez, right, originally from Mexico, watches with Yenny Quispe, center, who is from Peru and recently received her Green Card, during a watch party for President Barack Obama's speech on immigration on Jan. 29, 2013 in New York City.

By Carrie Dann, Political Reporter, NBC News

The debate over how to deal with the approximately 11 million individuals living in the United States without authorization - including the argument over whether to call them ?illegal? or ?undocumented? - is perhaps the most politically tricky aspect of the sprawling immigration policy overhaul effort.

So who are the 11 million? And how do we know how many there are?

It?s difficult to count people who by definition are unlikely to disclose their actual immigration status to the government, so demographers use what?s called the ?residual method? to determine about how many undocumented individuals are in the country.

Starting with Census Bureau data, the Pew Hispanic Center examines the total number of foreign-born individuals in the United States and subtracts those whose records or characteristics indicate they are here legally as naturalized citizens, Green Card holders, residents on temporary visas, or refugees.

?For those who say they are not a U.S. citizen and that they are foreign-born, we can, by looking at other characteristics -- like how long they have lived in the country and what job they hold -- determine whether the person is in the country legally or not,? says Mark Lopez, the associate director at the Pew Hispanic Center.??

The ?residual? means those who are left over.

Census data tends to?under-represent?certain groups, so Pew and others also try to fill in the gaps by adjusting for Census?under-counts. Demographers also factor in departure data like the number of deportations and apprehensions at the border.

Based on those demographic calculations, Pew estimated in 2011 that there are 11.1 million undocumented immigrants currently in the United States.

That number is statistically unchanged from estimates in 2010 and 2009, but has dropped significantly since 2007, when it spiked at 12 million.

Also in 2011, the Department of Homeland Security?s Office of Immigration Statistics?placed the number at 11.5 million, slightly higher than the Pew study.?

Another study by former U.S. Immigration and Naturalization Service demographer Robert Warren and University of Minnesota professor John Robert Warren pegged the total at around 11.7 million in January 2010. But all three data sets found a significant reduction in the population over the past decade.

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The Warren study concluded that, between 1990 and 2009, an estimated 7.5 million unauthorized immigrants left that population, either because they gained legal status, were removed by DHS, left voluntarily, or died.

Analysts attribute much of the decline since 2007 to the recession, particularly the burst of the U.S. housing bubble.??

?The Great Recession had a big impact, particularly on unauthorized immigrant workers, many of whom were in construction,? Lopez notes. ?So, many of them may have returned home.?

Advocates for undocumented immigrants emphasize that, while the stereotype of the ?illegal Mexican construction worker? has some basis in reality, that?s hardly the whole picture of the population.

According to DHS, while younger undocumented immigrants are more likely to be male, women make up 47 percent of the total undocumented population and a majority of those older than 45.

And, while about 1.6 million undocumented immigrants have arrived in the United States since 2005, a majority of them -- 56 percent -- first came to the country before 2000.

(While it is difficult to calculate how many of those undocumented immigrants entered the country via illegal border crossing versus how many came on a visa that expired, Pew estimated in 2006 that about 45 percent of new undocumented immigrants were in the latter category.)

Larry Downing / Reuters

Latinos protest in favor of comprehensive immigration reform on the West side of Capitol Hill in Washington, April 10, 2013.

Pew hasn?t done a deep data dive on the 2011 data, but its in-depth analysis of 2010 numbers showed that Mexicans made up 58 percent of the undocumented population. Individuals from other Latin American nations account for another 23 percent, and Asians for 11 percent.

Those numbers are similar to the findings from DHS, which found that individuals born in Mexico, Guatemala and El Salvador made up a combined 70 percent of the undocumented population in 2011.

While each data set uses slightly different methodology and yields slightly different estimates, analysts say the most important data point for public policy isn?t the overall number of undocumented immigrants, but the trends that show a decrease in the population overall.

"There may be some fluctuation in the numbers but what?s most important are the trends,? says Jeanne Batalova, a demographer at the Migration Policy Institute. ?The number definitely is not growing as fast as it used to be.?

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