In 2001, the Center for Impact Research conducted interviews with 222 women who currently or recently worked as prostitutes in the Chicago metropolitan area. By virtue of the study's methodology, this sample consisted only of women who were able and willing to meet with interviewers. Among these women, 61% said they entered prostitution before the age of 18, and 35% said they entered prostitution before the age of 16. Also, 59% said they could leave prostitution without being physically harmed, 27% said they could not, and 14% did not answer the question. Likewise, a 2011 study of 460 sex trafficking victims in the U.S. found that 54% of them were below the age of 18. Another study found that a significant portion of such victims are 11-13 years old.