As I understand it once a person is executed there is normally no way of demonstrating their innocence because lawyers etc. move on.
On the other hand there have been many post conviction DNA exonerations. There are figures here.
The Innocence Project - News and Information: Fact Sheets
From your source.
This I understand and have no quarrels with.
In more than 25 percent of cases in a National Institute of Justice study, suspects were excluded once DNA testing was conducted during the criminal investigation (the study, conducted in 1995, included 10,060 cases where testing was performed by FBI labs).
This on the other hand is a loop hole for releasing possible guilty inmates.
22 percent of cases closed by the Innocence Project since 2004 were closed because of lost or missing evidence.
Over the course of 10, 20, 30 years evidence gets lost, stolen, misplaced, mislabeled, etc. What can be done about it? It sounds simple on paper to keep evidence stored properly but if you have ever worked in a warehouse or anything of the like then you know that stuff gets misplaced and lost just due to human error.
Here is another loop hole created by the alleged murderers themselves. They falsely confess.
False confessions and incriminating statements lead to wrongful convictions in approximately 25 percent of cases.
Like I said the justice system is not perfect (human error), but then again nothing is. I would rather take the loss of an innocent man being executed wrongly then have a murderer slip away and kill freely.