WASHINGTON, D.C. – At least 4.1 percent of defendants sentenced to death in the United States are innocent, according to new peer-reviewed research published Monday (April 28th) in the Proceedings of the National Academy of Sciences, one of the world's most respected and cited scientific journals.
"(N)o process of removing potentially innocent defendants from the execution queue can be foolproof. With an error rate at trial over 4 percent, it is all but certain that several of the 1,320 defendants executed since 1977 were innocent," the study concludes. The article, "Rate of False Conviction of Criminal Defendants who are Sentenced to Death," is available at: http://www.pnas.org/cgi/doi/10.1073/pnas.1306417111.
"This study provides the first rigorous estimate of the rate of conviction of innocent criminal defendants in any context. It shows that the number of innocent people sentenced to death is more than twice the number of inmates actually exonerated and freed by legal action," said Bruce Levin, Ph.D., an expert in statistics who did not participate in the research but is familiar with the study. Dr. Levin is professor and past chair, Department of Biostatistics, Mailman School of Public Health Columbia University, and author of "Statistics for Lawyers" (2001, with Michael O. Finkelstein); "Statistical Methods for Rates and Proportions," 3rd Edition (2003, with Joseph L. Fleiss and Myunghee Cho Paik); and "The Biostatistics of Aging" (2014, with Gilberto Levy).
Since 1973, 1.6 percent of those sentenced to death in the U.S. – 138 prisoners – have been exonerated and released because of innocence. But many other innocent capital defendants are missed.
"The great majority of innocent people who are sentenced to death are never identified and freed. The purpose of our study is to account for the innocent defendants who are not exonerated," said Professor Samuel R. Gross of the University of Michigan Law School, the lead author of the study.
A major reason that many innocent capital defendants are not exonerated is that they are removed from death row but remain in prison. Lawyers, courts and governors focus a great deal of attention and time on reviewing the cases of prisoners who are on death row in order to reduce the risk of executing innocent people. But most death row prisoners have their sentences reduced to life in prison after appeals.
When that happens, the threat of execution is removed and far less time and resources are devoted to seeking out cases of possible innocence. As a result, an innocent defendant who might well be exonerated and released if he remained on death row for many years is likely to die in prison if he is resentenced to life.
To address this issue, the researchers employed a statistical technique called survival analysis that is commonly used in medicine. Survival analysis produces an estimate of the percentage of death-sentenced defendants who would be exonerated if they all remained indefinitely on death row and therefore subject to the exacting process for identifying innocent defendants that is applied to those who remain under threat of execution. The researchers concluded that their finding – 4.1 percent – is "a conservative estimate of the rate of false conviction among death sentences in the United States."
In addition to Gross, the authors of the study are: professor Barbara O'Brien, Ph.D., Michigan State University College of Law; Chen Hu, Ph.D., senior biostatistician, American College of Radiology Clinical Research Center and adjunct scholar, Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine; and Edward H. Kennedy, Department of Biostatistics and Epidemiology, University of Pennsylvania School of Medicine.