The data isn't perfect. Even Marcelli, whose report helps guide the estimate, told The Wall Street Journal in 2010 that he didn't think his numbers -- which look at Mexicans living in Los Angeles -- should be applied to a national formula, but agreed that demographers do not have any other empirical evidence at the moment with which to proceed.
Passel says that if considered along with several other types of data -- including the Mexican population figures -- demographers can get pretty close to a solid answer.
Still, Pew factors in a margin of error ranging from 300,000 to 500,000.
Although demographers seem to be getting better at estimating the number of undocumented, a mass legalization program in 2013 could provide an affirmation that the current methods are on track. As it stands, the current estimates still inspire debate, but they're much better than the numbers that were floating around in the early 1980s.