Cell block in Baghdad Central Prison (Abu Ghraib, Iraq)

About one in 25 inmates sentenced to death in the United States was likely wrongly convicted, a study said Monday.

Estimating the rate of false convictions, which the study put at 4.1 percent, is no easy task since there is no central database and many are never identified, in part because some sentences are commuted.

Nevertheless, "false convictions are far more likely to be detected among those cases that end in death sentences than in any other category of criminal convictions," said the article published in the Proceedings of the National Academy of Sciences.

"Everyone, from the first officer on the scene of a potentially capital crime to the Chief Justice of the United States, takes capital cases more seriously than other criminal prosecutions—and knows that everybody else will do so as well."

The study, the first of its kind, used a statistical method known as survival analysis. The method is usually used to determine the effectiveness of a medical treatment in reducing mortality rates.

Researchers were thus able to estimate the proportion of death row inmates whose innocence would have been established if they had stayed in prison and thus benefited from resources to defend themselves.

"Even if you are sentenced to death... the chance to be exonerated is much higher if you're still on death row," lead author Samuel Gross told AFP.

However, "the great majority of innocent people who are sentenced to death are never identified and freed."

The researchers used data from the 7,482 people sentenced to death from January 1974 to December 2004.

Among that group, 12.6 percent were executed, 1.6 percent were exonerated, four percent died while on death row, 46.1 percent remained on death row and 35.8 percent were taken off death row but stayed in prison after their capital sentences or convictions were reversed or changed.

Based on the analysis showing a more than four percent error margin in trials, the study said it was "all but certain" that several of the 1,320 people executed since 1977 were in fact innocent.

"Most innocent defendants who have been sentenced to death have not been exonerated, and many—including the great majority of those who have been re-sentenced to life in prison—probably never will be," it added.

"The net result is that the great majority of innocent defendants who are convicted of capital murder in the United States are neither executed nor exonerated. They are sentenced, or re-sentenced to prison for life, and then forgotten."

Explore further Exonerations correct only a small fraction of false convictions

More information: Rate of false conviction of criminal defendants who are sentenced to death, PNAS, 2014. Journal information: Proceedings of the National Academy of Sciences Rate of false conviction of criminal defendants who are sentenced to death,, 2014. www.pnas.org/content/early/2014/04/23/1306417111

© 2014 AFP