If the study's conclusion is accurate, it means the number of people who have been exonerated of the crimes for which they were sentenced to be executed would be nearly double the number who actually have been. And the authors make clear that their estimate is conservative. Since 1989, 138 death row inmates have been exonerated after proving they were innocent. The authors estimate that 340 would be exonerated under the conditions they set forth.
Here's the abstract of "Rate of false conviction of criminal defendants who are sentenced to death" by Samuel R. Gross, Barbara O’Brien, Chen Huc and Edward H. Kennedy:
The rate of erroneous conviction of innocent criminal defendants is often described as not merely unknown but unknowable. There is no systematic method to determine the accuracy of a criminal conviction; if there were, these errors would not occur in the first place. As a result, very few false convictions are ever discovered, and those that are discovered are not representative of the group as a whole. In the United States, however, a high proportion of false convictions that do come to light and produce exonerations are concentrated among the tiny minority of cases in which defendants are sentenced to death. This makes it possible to use data on death row exonerations to estimate the overall rate of false conviction among death sentences. The high rate of exoneration among death-sentenced defendants appears to be driven by the threat of execution, but most death-sentenced defendants are removed from death row and resentenced to life imprisonment, after which the likelihood of exoneration drops sharply. We use survival analysis to model this effect, and estimate that if all death-sentenced defendants remained under sentence of death indefinitely, at least 4.1% would be exonerated. We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States.In taking note of the study, The Innocence Project points out that "many other innocent capital defendants are missed if they are removed from death row or have their sentences reduced to life in prison after appeals. Once the threat of wrongful execution is removed, less time and resources are devoted to seeking out cases of possible innocence."
Please read below the fold for more on this subject.
The authors found that some 2,675 people were taken off death row between 1973 and 2004 and resentenced because of doubts about their convictions. Many have since died and most of the rest will die in prison without our ever learning whether they committed the crimes they were once sentenced to die for. It is forever impossible to know how many people were falsely convicted. But lead author Samuel Gross says:
“If you look at the numbers in our study, at how many errors are made, then you cannot believe that we haven’t executed any innocent person – that would be wishful thinking.” [...]One of the more charitable descriptions of Scalia's remarks.
The ballpark figure of at least 4.1% innocence is higher than previous studies looking at exoneration rates that had smaller sample sizes and were more restricted in their remit. It is also considerably higher than the estimate given in 2007 by the conservative US supreme court justice Antonin Scalia, who wrote that American criminal convictions generally had an “error rate of .027% – or, to put it another way, a success rate of 99.973%”.
The authors comment tartly with respect to Scalia’s skills as a statistician: “That would be comforting, if true. In fact, the claim is silly.”