The error rate of supposition times extrapolation times wishful thinking exceeds the result. This field is full of the most persuasive and motivated lawyers on the planet. Doubt everything.To approximate a number that has long been considered unknowable, the authors studied the subset of prisoners who, if wrongfully convicted, are most likely to be exonerated: those facing the threat of execution. Because the justice system goes to lengths not to execute innocent people, the cases of prisoners on death row receive far more scrutiny and go through many more appeals processes, raising the likelihood that any flaws in their convictions are found.
"The result is an exoneration rate that is far, far higher than for other [types of] cases, hundreds of times higher than for felons in general," Gross explained. "That means that it might be possible to use that very high exoneration rate as a good measure of the underlying rate of innocence, because it might come close to discovering all the cases that are there."
The study required some inventive math, however, because the highest predictor of being exonerated is not merely being sentenced to death but being currently under the threat of death. Many people sentenced to death ultimately have their sentences reduced to life in prison, at which point the criminal justice system stops focusing on their cases and their chances of being exonerated drops dramatically.
In order to find the innocence rate, the study had to determine the probability that someone sentenced to death would be exonerated if he or she remained under the threat of execution indefinitely. So Gross and O'Brien teamed up with biostatistician Chen Hu from the University of Pennsylvania School of Medicine and applied "survival analysis," a technique generally used in medicine, to find the likelihood that someone on death row is innocent.
Edit: I should flesh this out. Capital crimes have more lawyers on both sides, fighting diligently. The innocence project is just one of dozens of anti capital offence organizations out there. They do good work, as I said, I'm not a fan of the death penalty. But, they tend, like all True Believers, to want to convince, more than they want to educate. We really don't execute many people. 20 people last year. That's less than 40% of the deaths caused by lightning. If I were to get worked up about innocent people killed in our justice system, I expect more are killed by prison population, than by prison guards. If I were to get worked up about state caused deaths, I would focus on keeping our soldiers at home. It's not that I am not concerned by the death penalty, but that it is a relatively small concern, for me.
My solution isn't very original, brute already brought it up. Saving prison for criminals, rather than filling them with unlicensed merchants just seems reasonable.