 The users of this methodology assume that 10% of undocumented immigrants are not counted by census takers.The 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