"Researchers use federal data going back to the early 1990s to track how student borrowing has changed over time, but adjustments made to the most recent federal study of student aid render such comparisons inaccurate - for now," Chronicle of Higher Education reports. "For years, the National Postsecondary Student Aid Study calculated student debt based on loan commitments, or essentially how much money students had been promised, rather than disbursements, or how much they actually borrowed. But with the 2007-8 study, the National Center for Education Statistics, the Education Department’s statistical arm, decided to calculate the loan data based on how much students actually took out, according to a notice posted on its Web site. The change made it impossible to compare the most recent data to past studies."