In a third of all states, low-income students need loans even to attend some two-year community colleges, according to the study, which was released Monday.
The findings of the year-old Lumina Foundation for Education have sparked sharp criticism from higher education groups.
The foundation rated nearly 3,000 colleges and universities, and said that while at least half the public four-year schools in 40 states are financially manageable for median-income students, those students often need loans.
Only in Alaska, Arkansas, Hawaii, Kentucky and Wyoming are all four-year public colleges affordable for low-income people, it said.
Critics complained that the study flies in the face of reality: 15 million people from all income levels attend college at two- and four-year schools. They also charged that the study risks discouraging those who might benefit most from a college degree.
Lumina's vice president for research, Jerry Davis, said the study focuses on the hardships imposed by paying for college.
"We're saying students and families must make inordinate financial sacrifices to attend those schools," Davis said. The struggle to afford college leads some to quit, he said.
Davis said he had hoped that higher education officials would use the study to help secure more state and federal aid for students.
The study arrives as the recession is both driving up demand for college - as people look to improve their skills and resumes - and the cost of attending, especially at state institutions where about 80 percent of college students are found.
The study used 1998 federal statistics on income, enrollment and financial aid, among other factors. It looked at four income groups: low- and median-income students still dependent on parents' income, and independent students ages 25-34 with low or median incomes.
Higher education groups said the study's methods were flawed and could put people off the idea of college or certain institutions.
"Enrollments go up every single year," said Terry Hartle, vice president of the American Council on Education. "If this is correct, there are a lot of people in higher education that aren't supposed to be there."
Hartle lauded Lumina's effort but said it would reinforce mistaken assumptions. Surveys find the public tends to overestimate the cost of a college education, he said.
David Warren, head of the National Association of Independent Colleges and Universities, said the report "misrepresents reality, misleads readers, and harms the very families the foundation is trying to help."
The topic of cost is "probably one of the touchiest policy issues in higher education right now," said Travis Reindl, state policy director at the American Associaion of State Colleges and Universities.
Still, Reindl said it was unfair for the study to label specific schools as "unaffordable."
"If you're going to really judge an institution, you have to really dig into the nitty-gritty of what's happening at the institution," he said.
The nonprofit Lumina Foundation was created with proceeds from the 2000 purchase of USA Group, a nonprofit company that services student loans, by Sallie Mae, a leading provider of student loans. The foundation is devoted to expanding access to higher education.
© MMII The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed