More than 4 per cent of inmates sentenced to death on the United States’ notorious death row are probably innocent, research suggests.

The findings, led by a University of Michigan Law School professor, offer a “conservative estimate” of the number of wrongfully convicted death row inmates over three decades.

Researchers reviewed the outcomes of the 7,482 death sentences issued from 1973 to 2004, and found that of that group, 117 inmates were exonerated.

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They concluded that with enough time and resources, more than 200 other prisoners, at least 4.1 per cent, would have been proved innocent.

They reached that conclusion by using survival analysis, an advanced statistics tool used in medicine to judge the effectiveness of new treatments.

One of the main reasons why wrongly convicted defendants are not vindicated is because many win appeals reducing their death sentences to life in prison.

Researchers believe that once they have been granted a life sentence, proving their innocence is not as thoroughly pursued.

The results of the probe are likely to send shock waves through anti-death penalty campaign groups.

Richard Dieter, executive director of the Death Penalty Information Centre, said the findings expose profound problems with the death penalty.

“This impressive study points to a serious flaw in our use of the death penalty,” he said. “The ‘problem of innocence’ is much worse than was thought."

The article was published yesterday in an American academic journal.