On January 20, 2016, the Center for Migration Studies of New York (CMS) released a report finding that the total undocumented population in the United States has declined gradually over the past few years. The day before, the US Department of Homeland Security (DHS) released a report titled “Entry/Exit Overstay Report: Fiscal Year 2015,” which concluded that by the end of FY 2015, there were 482,781 “Suspected In-Country Overstays;” that is, individuals who remained in the United States beyond their period of admission and DHS had “no evidence of a departure or transition to another immigration status.” CMS previously reported that visa over-stayers now represent the majority of non-citizens joining the undocumented population each year. National Public Radio and others have asked whether DHS visa overstay report affected the validity of CMS’s finding related to the decline in the undocumented population.
In fact, DHS’s findings on overstays and CMS’s estimates on the undocumented complement each other. Although precise figures are not available, large numbers of visa over-stayers from past years invariably leave the nation in subsequent years. Thus, one of the most important overstay statistics is the total overstay population, which equals the total number of (carryover) over-stayers from the previous year (year one), minus those who leave the overstay population during the year (year two), plus those who join the overstay population during the year. The DHS’s overstay statistic of 482,781 refers to the number of new overstays during the year, and does not account for departures in FY 2015 of over-stayers from earlier years. Likewise, CMS’s statistics on the percentage of new overstay “arrivals” into the unauthorized population – 58 percent in 2013 – refers to overstays during the year. In short, neither statistic refers to the total overstay population.
In addition, the DHS report likely overestimates the number of overstays in FY 2015 from select countries. The DHS statistics refer only to departures that occur from air or sea ports-of-entry, and not land ports-of-entry. The CMS estimates show that only 1,100 undocumented immigrants per year entered from Canada over the past five years, but the DHS report indicates that 93,035 Canadians overstayed non-immigrant visas in FY 2015. It is not surprising that the DHS report overestimates Canadian overstays. As noted by GAO (1995), the collection of information for Canada and Mexico is especially problematic: “Notably, Mexicans and Canadians who arrive by air may be more likely to depart by land than visitors from countries that do not border the United States.” In other words, Canadians would be counted as “overstays” even though they departed by land in disproportionate numbers. As another example, DHS estimates of German over-stayers in FY 2015 (22,554) substantially exceed CMS’s estimates for undocumented entries from Germany in recent years. Despite significant improvements in reporting departures by commercial and private carriers in recent years, information on arrivals is more likely to be complete than information about departures because DHS inspects all persons who are arriving. In testimony on the overstay report, DHS officials touted “near 100 percent” departure reporting, but also referenced fines for non-compliance on a monthly basis.
The recent CMS and DHS reports cover different years, and CMS lacks data on the undocumented population in FY 2015. However, CMS estimates show that at least 350,000 undocumented residents who had entered the country from 1982 through 2012 left the undocumented overstay population in 2013. CMS’s estimates of departures from the overstay population in recent years, combined with its finding that DHS overestimated overstay numbers in FY 2015 for several countries, suggest that more visa over-stayers may have left the undocumented population in recent years than joined it. Of course, this finding would be consistent with a declining undocumented population. CMS plans to release a more detailed essay on these issues in the near future.
 The DHS report defines an “overstay” as a “nonimmigrant who was lawfully admitted to the United States for an authorized period but stayed or remains in the United States beyond his or her lawful admission period.”
 The DHS report covered only non-immigrant visitors for business or pleasure, and who entered through an air or sea port-of-entry.
 The DHS report points out that by January 4, 2016, the number of people who had overstayed during FY 2015 and who remained in the United States had dropped to 416,500.
DHS (US Department of Homeland Security). 2016. U.S. Entry/Exit Overstay Report Fiscal Year 2015. http://www.dhs.gov/publication/entryexit-overstay-report.
GAO (US Government Accountability Office).1995. Illegal Immigration: INS Overstay Estimation Methods Need Improvement. GAO/PEMD-95-20. http://www.gao.gov/products/PEMD-95-20.
Warren, Robert. 2016. “US Undocumented Population Drops Below 11 Million in 2014, with Continued Declines in the Mexican Undocumented Population.” Journal on Migration and Human Security 4(1):1-15. https://doi.org/10.1177/233150241600400101.