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About 4.1 percent of those who were sentenced to death between 1973 and 2004, or about 300 people, are likely innocent, according to new analysis published in the Proceedings of the National Academy of Sciences on Monday. 

Courtesy of PNAS

To reach their conclusion, researchers relied on "survival analysis," a statistical method of predicting the amount of time it will take for an event to take place, which is used to predict rates of death, disease and other like occurrences in a large sample.

The paper authors, led by Samuel R. Gross, explain that the number of innocent people sentenced to death does not correlate to the amount actually executed, which is much lower. They write in the study's abstract:

The high rate of exoneration among death-sentenced defendants appears to be driven by the threat of execution, but most death-sentenced defendants are removed from death row and resentenced to life imprisonment, after which the likelihood of exoneration drops sharply.

In the years covered in the study, 7,482 people were sentenced to death. Of those, 943 were executed and 117 exonerated. The rest either died on death row, but were not executed; remain on death row now; or were removed from death row without having been exonerated.  In the paper, the authors go on to say that the high rate of exonerations in recent years suggests that the rate of innocence among those given the death penalty is also high: 

In the past few decades a surge of hundreds of exonerations of innocent criminal defendants has drawn attention to the problem of erroneous conviction, and led to a spate of reforms in criminal investigation and adjudication. All the same, the most basic empirical question about false convictions remains unanswered: How common are these miscarriages of justice?

The Washington Post explains the criminal justice implications of Gross's work: 

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.

The authors conclude that their estimate of 4.1 percent is likely too low, and take issue with the opinion that death sentences are doled out correctly. They write that "In 2007, Antonin Scalia wrote in a concurring opinion in the Supreme Court that American criminal convictions have an 'error rate of .027 percent—or, to put it another way, a success rate of 99.973 percent.'" This, the authors say, "would be comforting, if true."

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