WASHINGTON ― Science and law have led to the exoneration of hundreds of criminal defendants in recent decades, but big questions remain: How many other innocent defendants are locked up? How many are wrongly executed?
About one in 25 people imprisoned under a death sentence is likely innocent, according to a new statistical study appearing in the Proceedings of the National Academy of Sciences. And that means it is all but certain that at least several of the 1,320 defendants executed since 1977 were innocent, the study says.
From 1973 to 2004, 1.6 percent of those sentenced to death in the U.S. ― 138 prisoners ― were exonerated and released because of innocence.
But the great majority of innocent people who are sentenced to death are never identified and freed, says professor Samuel Gross of the University of Michigan Law School, the study’s lead author.
The difficulty in identifying innocent inmates stems from the fact that more than 60 percent of prisoners in death penalty cases ultimately are removed from death row and resentenced to life imprisonment. Once that happens, their cases no longer receive the exhaustive reviews that the legal system provides for those on death row.
Gross and three other researchers, including a biostatistics expert, looked at the issue using a technique often used in medical studies called survival analysis. Yale University biostatistics expert Theodore Holford, who wasn’t part of the study, said the work done by Gross “seems to be a reasonable way to look at these data.”
Because of various assumptions, it might be best to use the margin of error in the study and say the innocence rate is probably between 2.8 percent and 5.2 percent, said University of South Carolina statistics professor John Grego, who wasn’t part of the study.
