A recent Mother Jones article attempts to answer this question with help from the Innocence Project, the Center on Wrongful Convictions and experts in the field.
Extrapolating from the 281 known DNA exonerations in the US since the late 1980s, a conservative
estimate is that 1 percent of the US prison population, approximately 20,000 people, are falsely convicted.
In fact, since the late 1980s there have been as many as 850 exonerations nationwide, according to
University of Michigan law professor Samuel Gross, a leading researcher in the field. Many of them float under the radar, Gross says, unlike the highly publicized DNA exonerations.
Read the full article and see a map
of how many people have been exonerated in each state since 1989.
See how the Innocence Project has answered this question.
Browse case profiles of DNA exonerations.
How does an individual help?
I’d really like a failure rate of the court system though. 97% of federal and 94% of state cases results in plea deals. If you were innocent and didn’t take your chance to go through a trial, you sitting in jail is really your fault at that point. Of the 3% federal and 6% state trials that end in a guilty verdict, when the accused is innocent, I think is the fail rate of the process. So take 1000 federal cases for instance. 30 of them go to trial. 970 cases should be excluded in calculating if the process is fair since they didn’t go through it till the end. Can I judge how good a car wash cleaned my car if I bail out when I only went through 3/4 of the process? So, how many of the 30 trials were found guilty, and of those, what percent was actually innocent? Measuring how many innocent people plead guilty seems to be more of a human fail rate than the justice system’s fail rate.