One in 25 death row inmates in US likely innocent, study finds

Science and law have led to the exoneration of hundreds of criminal defendants in the US 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 people executed since 1977 were innocent, the study says.

From 1973 to 2004, 1.6 per cent of those sentenced to death - 138 prisoners - were exonerated and released because of innocence.

But most innocent people 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 about 60 per cent of prisoners on death row are 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".