Some 13.5 million Americans have been homeless for at least a few days sometime in their lives, and an additional 12.5 million have stayed off the street only by moving in with friends or family, Columbia University researchers contend.

The new study provides the highest estimate yet of the prevalence of homelessness - and concludes that the biggest risk factor these millions had in common was low income.Thus, the study debunks beliefs that "homelessness is an aberration affecting Americans who have distinct personal histories or who are situated on the fringes of society," Yale University's Dr. Robert Rosenheck wrote in an editorial accompanying the study in today's American Journal of Public Health.

But the research, based on a telephone survey that asked 1,507 adults whether they recalled ever being homeless, drew prompt criticism.

Michael Horowitz of the Hudson Institute, a conservative think tank, called the numbers overblown. People could easily have recalled incorrectly, misinterpreted a night or two between homes or defined homelessness as a brief teenage runaway or even a hitchhiking trip, he said. "They've got to be phenomenally suspect, these numbers," he said.

Estimates of homelessness fluctuate greatly. The 1990 Census said 400,000 Americans were homeless while advocacy groups have put the number between 700,000 and 3 million. Last May, the Clinton administration said 600,000 Americans may be homeless on any given day.

But most of those estimates are based on "snapshot" tallies of people in shelters and on the street that give nightly totals but can't tell for how long people are homeless.

Columbia professor Bruce Link tried to better determine the prevalence of homelessness by asking whether his subjects recalled ever being homeless, for how long and where they slept - in the street, a shelter, abandoned buildings or someone else's home.

His conclusion: 26 million U.S. adults have experienced some form of homelessness, including being forced to live with someone else, and 13.5 million of those have been forced to live in shelters or on the streets.

Among those who said they had experienced homelessness, 46 percent said they had been homeless between a month and a year, 33 percent between a week and a month, 13 percent for more than a year and 8 percent for less than a week.

The most likely risk factor was poverty, he said. Less than 10 percent of the formerly homeless earned more than $20,000 a year.

Horowitz argued that studies have shown about half of all homeless people have serious mental illnesses, addictions or other prob-lems. Research that has contended that larger segments of the population are homeless has later been found to have been flawed, he added.

But Link said his numbers may be low because he only surveyed people with telephones.