A psychologist's experiment raises questions about the accuracy of eyewitnesses' identification of suspects, which are used more than 75,000 times a year in the United States to charge people with crimes and prosecute the cases.

The experiment showed that people who identify a suspect from a police lineup or a group of photos are far more confident of their choice if given positive feedback, even in casual conversation.They become less sure of the identification - and likely to be less-confident witnesses at trial - if given negative feedback or none at all.

In research being published by the American Psychological Association, 352 people were shown a grainy videotape made by a surveillance camera of a person who later shot and killed a store security guard. They were told the person they saw had shot a security guard and were shown either a lineup or photographs and asked to identify the killer. In fact, he was in neither the lineup nor the photos.

All made a choice - wrong, of course, since they had not seen the gunman.

Those who were told, "Good, you identified the actual suspect," were far more confident in their choices than those told the suspect actually was among the other people shown. Those given positive feedback also were more confident than those who were told nothing about whether they had fingered the right man.

"In addition to being more confident of their choice of photographs, they also remembered having a better view of the culprit, having paid greater attention to the videotape, having had an easier time making the identification, needing less time to make the identification and being better able to make out details of the culprit's face than those who received negative feedback or no feedback," the association reported.

The experiment by Iowa State University psychologist Gary L. Wells and his assistant, Amy L. Bradfield, is recounted in the June issue of the association's Journal of Applied Psychology, released Sunday.

Former New York City police commissioner Patrick Murphy, director of the police policy board at the U.S. Conference of Mayors, said in an interview that good police departments train detectives not to talk to witnesses about the identifications they make.

"In my experience, a detective who can keep that objectivity will end up with a better witness than one who is encouraging the witness," Murphy said.

Wells said statistics are not kept on how often eyewitnesses are told anything after picking out a suspect.

"We only know that it iscommon because it often appears in trial transcripts when police are carefully questioned about what they told the eyewitnesses," Wells said.

He said stronger feedback - "That's the guy!" or "This will allow us to finally get this guy off the street" - could have an even greater effect on the eyewitnesses' certainty.

Lawyer Lawrence Vogelman of Exeter, N.H., a director of the National Association of Criminal Defense Lawyers, said he was not surprised by the findings.

"Witnesses don't want to be wrong," he said. "They're afraid of making mistakes, so if they get any indication that the decision they made is `correct,' they are more comfortable with it."

Wells cited research published in 1989 that said more than 75,000 people a year become criminal suspects based on having been picked out by eyewitnesses from lineups or photo spreads.

"The identification of innocent persons . . . is the primary cause of wrongful conviction, accounting for more convictions of innocent persons than all other causes combined," he said.

In a 1996 study by the Institute for Law and Justice of 28 people released from long prison terms on the basis of DNA tests, 24 had been misidentified by eyewitnesses, he said.

The researchers cited testimony from a woman who identified her attacker from a lineup of four people.

At the lineup, she had this to say: "I don't know . . . Oh, man. . . . It's one of those two, but I don't know." Thirty minutes later, she said: "I don't know. . . . No. 2?"

But at the trial she said: "There was no maybe about it. . . . I was absolutely positive."

Wells wrote that "one possibility" for her positive testimony was that she had been the recipient of positive feedback.