The study identified 199 murder exonerations, 73 of them in capital cases. It also found 120 rape exonerations. Only nine cases involved other crimes. In more than half of the cases, the defendants had been in prison for more than 10 years. The study's authors said they picked 1989 as a starting point because that was the year of the first DNA exoneration. Of the 328 exonerations they found in the intervening years, 145 involved DNA evidence. In 88 percent of the rape cases in the study, DNA evidence helped free the inmate. But biological evidence is far less likely to be available or provide definitive proof in other kinds of cases. Only 20 percent of the murder exonerations involved DNA evidence, and almost all of those were rape-murders.

As to the cause of wrongful convictions, the study finds in 90% of the rape cases, faulty eyewitness testimony was responsible--particularly where cross-racial identification was involved.

In particular, the study said black men made up a disproportionate number of exonerated rape defendants.....while 29 percent of those in prison for rape are black, 65 percent of those exonerated of the crime are.

The causes of wrongful murder convictions were different:

On the other hand, the study found that the leading causes of wrongful convictions for murder were false confessions and perjury by co- defendants, informants, police officers or forensic scientists.

As to false confessions,

"There are three groups of people most likely to confess," said Steven A. Drizin, a law professor at Northwestern, who conducted the study with Richard A. Leo, a professor of criminology at the University of California, Irvine. "They are the mentally retarded, the mentally ill and juveniles."

It's not just death row inmates who get exonerated. It's that death row cases get more scrutiny. Extrapolating from there, the researchers concluded:

Death row inmates represent a quarter of 1 percent of the prison population but 22 percent of the exonerated.That suggests that innocent people are often convicted in run-of-the-mill cases. Indeed, the study says, "if we reviewed prison sentences with the same level of care that we devote to death sentences, there would have been over 28,500 non-death-row exonerations in the past 15 years rather than the 255 that have in fact occurred."

Jeers for the day go to prosecutor Joshua Marquis in Washington who said:

""We all agree that it is better for 10 guilty men to go free than for one innocent man to be convicted," Mr. Marquis said. "Is it better for 100,000 guilty men to walk free rather than have one innocent man convicted? The cost-benefit policy answer is no."

Wrong formula. Here's the right one, from Prof. Samuel Gross at the University of Michigan:

"No rate of preventable errors that destroy people's lives and destroy the lives of those close to them is acceptable,"

Innocence Project co-founder Barry Scheck points out another weakness in Marquis' view:

"Every time an innocent person is convicted," Mr. Scheck said, "it means there are more guilty people out there who are still committing crimes."