10% assumption is based on a 2001 University of California survey asked of 829 people born in Mexico and living in Los Angeles whether they responded to census interviewers in the 2000 census with 40% of queried households refusing to answer the survey. Critics claim that the estimate is unreliable for a number of reasons: figures for outmigration are not tracked by the federal government; the proportion of undocumented immigrants who respond to the Census is unknown; the estimate that 10% of undocumented immigrants do not respond to the census is arbitrary and unsupported by a sufficient sample size and geographic spread; and that the self reporting of where one was born relies on the honesty of the person being questioned. In 2016, the Pew Research Center estimated that, in 2014, there were 11.1 million undocumented immigrants in the US. These estimates are based on modeling using data from the American Community Survey (ACS) or the Current Population Survey conducted by the United States Census Bureau each year. A 2018 paper by three Yale School of Management professors yielded similar trajectories of the undocumented immigrant population, with peak growth in the 1990s and early 2000s followed by a plateau from approximately 2008 onward. However, their model yielded an estimate of the numbers of undocumented immigrants of 22.1 million undocumented immigrants as the mean — roughly twice as great as the estimates based on the ACS. Moreover, according to the three authors, their estimate has a 95 percent probability range of 16 million to 29 million. That result, however, was criticized for vastly overstating the true number and for failing to account for the circular flow rate. Impact of the global financial crisis of 2008–2009 The 2008 global financial crisis had a large impact on the United States. The construction sector and other areas where undocumented immigrants traditionally seek employment shrank. The recession also led to a surplus of American labor, driving down the benefit of hiring undocumented immigrants. According to the Pew Research Center, in 2007 the number of unauthorized Mexican immigrants peaked at 6.9 million and has dropped by more than 1 million to an estimated 5.6 million in 2014. After the Great Recession, more immigrants actually returned to Mexico rather than migrated to the United States. From 2009 to 2014, 1 million Mexicans and their families left the U.S. for Mexico. U.S. census data for the same period