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Medical Statistical Model Used to Estimate Wrongful Conviction Rate in Death Penalty Cases

An interesting collaboration between medical and law professionals, under the leadership of University of Michigan Law School professor Samuel R. Gross, led to the application of medical statistical analysis to exonerations of death-sentenced prisoners, in order to estimate the number of innocent defendants who receive the death penalty. The report, published in April 2014 in the Proceedings of the National Academy of Sciences, the nation’s most prestigious scientific journal, estimated the rate of wrongful capital convictions at 4.1%, or 1 in 25. The 95% confidence level of the study was 2.8 to 5.2%. This was the first report to use “solid and appropriate statistical methods” to estimate the error rate in capital convictions.

Death penalty cases were studied because they are unique among criminal prosecutions, in that they receive more serious scrutiny at every step of the process from initial crime scene investigation to execution. Death-sentenced prisoners usually have the assistance of attorneys until the execution is carried out. Presumably, jurors take their task more seriously in capital cases, and judges apply greater scrutiny when a person’s life is at stake.

This additional scrutiny means the rate of exonerations of death-sentenced prisoners may be close to the actual rate of wrongful convictions overall. Going out of their way to be very conservative in their estimations, the authors found that the exoneration rate varied with length of incarceration, as one might expect. Prisoners who had been on death row for five years had an exoneration rate of 1%. At ten years it was 1.6%; at 15 years, 2.6% and at 21.4 years it reached 4.1%.

The study examined all of the 7,482 capital convictions from when the death penalty was reinstated in 1976 through 2004. Of those defendants, 35.8% had been resentenced to a term of imprisonment and 117 (1.6%) had been exonerated. Only 10 of the exonerations involved prisoners who had also been resentenced to a term of imprisonment. Over 12% of the defendants had been executed, while 4% had committed suicide and 41.6% remained on death row.

The authors of the report noted that two previous studies of exonerations, one of which was by the same authors, underestimated the rate of exonerations for death-sentenced prisoners because they did not take into account exonerations that occurred after the relatively short study period and did not include undetected wrongful convictions. The authors also noted that the exoneration rate is much greater for death-sentenced prisoners due to the greater scrutiny their cases receive. This is true even when a death-sentenced prisoner is resentenced to a term of imprisonment; such prisoners have exoneration rates 1/8 that of the prisoners who remain on death row.

Which is ironic because, as the report concludes, “those who are resentenced to punishments less than death are more likely to be innocent than those who remain on death row.” This may be because judges take innocence into account when deciding whether to reverse a conviction and again subject a defendant to the possibility of a death sentence, or to commute a death sentence to life imprisonment which spares the prisoner from execution but also greatly reduces the chances of a subsequent exoneration.

“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,” the study noted. “They are sentenced, or resentenced to prison for life, and then forgotten.”

The report concluded that even based on a conservative estimate, it was likely that several of the 1,320 defendants executed since 1977 were innocent.

In a July 2015 article published in the Washington Post, study leader Samuel Gross argued that wrongful convictions are the product of insufficient time and resources within the criminal justice system and occur at the expense of innocent people’s freedom. They also allow the guilty to avoid punishment and potentially commit more crimes.

“It’ll cost money to achieve the quality of justice we claim to provide: to do more careful investigations, to take fewer quick guilty pleas and conduct more trials, and to make sure those trials are well done. But first we have to recognize that what we do now is not good enough,” he wrote.

Sources:, Associated Press,

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