The Department of Homeland Security (DHS) issued in July 2013 a paper entitled “Estimates of the Legal Permanent Population in 2012.” (http://www.dhs.gov/sites/default/files/publications/immigration-statistics/ois_lpr_pe_2011.pdf) In it the author claims that there are 13.3 million Legal Permanent Residents (LPR – “green card holders”) among whom are 8.77 million LPRs are eligible to apply for U.S. citizenship on the basis of their length of residence in the United States. This finding is based on DHS records and Census Bureau data.
The finding is curious. The Census Bureau estimates there were 40.38 million foreign-born residents in the country in 2011 among whom 18.14 million were naturalized U.S. citizens. That leaves 22.24 million non-citizen foreign-born residents. If 13.3 million of those foreign-born residents are LPRs, that leaves less than 9 million foreign-born residents who are not LPRs. By definition, illegal aliens are not LPRs, and they are estimated by DHS at more than 11 million. In addition, there are millions of foreign nonimmigrants admitted for extended periods who are also considered part of the resident foreign-born population by the Census Bureau (intra-company transfers, high-tech workers, NAFTA professionals, etc.)
So there is a seeming discrepancy and a question as to where it may come from. One part of the explanation is the undercount of the illegal alien population by the Census Bureau. The Pew Hispanic Center has used a 10 percent undercount estimate, implying at most about 1.1 million fewer residents unaccounted for. The other possibility of error is with the DHS data. Because DHS does not keep comprehensive records on the immigrant population, it may be counting in the LPRs who have left the country. Although it seems unlikely, it is also possible that some of the LPR population tabulated by DHS have already become naturalized U.S. citizens. In any case, the implication of the DHS report that there are nearly 9 million LPRs eligible to naturalize as U.S. citizens should be taken with a grain of salt.