One in seven Vietnam veterans suffers from post-traumatic stress disorder, a rate seven times that estimated by a government study earlier this year, a researcher reported Saturday.

The 15 percent rate cited in the latest study translates to about 470,000 cases.The discrepancy with the 2.2 percent rate reported earlier by the federal Centers for Disease Control arises mostly from differences in the criteria for diagnosis.

In contrast to a single yardstick used in the CDC study, the new estimate relied on several measures, said researcher William Schlenger.

"To me the compelling reason for believing we're right is we have these multiple measures," said Schlenger, of the Research Triangle Institute in Research Triangle Park, N.C.

He described the study at the annual meeting of the American Psychological Association and in interviews.

Post-traumatic stress disorder can appear months or years after a traumatic experience such as combat, natural disaster or airplane crash.

Symptoms include sudden, vivid reliving of the experience, nightmares that can make a person awaken screaming, and intense distress from events that symbolize or resemble the trauma. Some victims remain on constant alert for danger.

The new estimate says the disorder afflicts 15 percent of male Vietnam veterans and 9 percent of female Vietnam veterans. The number of females was small enough that the overall rate is still 15 percent, Schlenger said.

The estimate was developed by researchers at Schlenger's institute, a private not-for-profit research firm; San Diego State University, and the University of California at San Francisco.

The "National Vietnam Veterans Readjustment Study" included long interviews with about 1,600 veterans who served in Vietnam, Laos or Cambodia or the surrounding waters or airspace.