A new study published in the Proceedings of the National Academy of Sciences uses statistical techniques to estimate that one in every 25 death row inmates in America would be exonerated, given enough time and attention.
The study builds on the data accumulated in the course of years of previous exonerations (examples of which are here) in order to make estimates about the death row population as a whole. Here is the abstract of the study, bolding ours:
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.
The gravity of the death sentence is what draws the scrutiny necessary to result in an exoneration. When the death sentence is out of the picture, the probability of exoneration drops. So for at least one in 25 death row inmates, the attention drawn by the possibility of capital punishment could be the only force potent enough to save them from capital punishment.
The system is not a good one.