As a result, the study’s authors note, “it is all but certain that several of the 1,320 defendants executed since 1977 were innocent.”
One hundred and thirty eight prisoners sentenced to death since 1973, or 1.6 percent of the total, have been exonerated and released because of innocence. But many other innocent capital defendants are missed, according to the study’s authors.
“The great majority of innocent people who are sentenced to death are never identified and freed. The purpose of our study is to account for the innocent defendants who are not exonerated,” said Professor Samuel R. Gross of the University of Michigan Law School, the lead author of the study.
The researchers employed a statistical technique called survival analysis, producing an estimate of the percentage of death-sentenced defendants who would be exonerated if they remained indefinitely on death row.