Using data from longitudinal studies conducted by the U.S. Department of Education, researchers Judy Goldberg Dey and Catherine Hill analyzed some 9,000 college graduates from 1992–93 and more than 10,000 from 1999–2000. The researchers controlled for a multitude of variables, including: occupation, industry, hours worked per week, workplace flexibility, ability to telecommute, whether employee worked multiple jobs, months at employer, marital status, whether employee had children, and whether employee volunteered in the past year. The study found that wage inequities start early and worsen over time. "The portion of the pay gap that remains unexplained after all other factors are taken into account is 5 percent one year after graduation and 12 percent 10 years after graduation. These unexplained gaps are evidence of discrimination, which remains a serious problem for women in the work force."