By By Brett Wilkins Apr 28, 2014 in Crime A newly-published study asserts about 4 percent of individuals currently sentenced to death in the United States is likely innocent, raising life-or-death questions about the morality of capital punishment. From 1973 until 2004, 1.6 percent of prisoners on America's death rows, or 138 individuals, have been exonerated and freed after proving their innocence. Samuel Gross of the University of Michigan Law School, the study's lead author, says the majority of wrongfully convicted and condemned prisoners are never exonerated or freed. Researchers attempting to determine inmate innocence have long been hamstrung by the fact that 60 percent of death row prisoners are eventually removed from death row and sentenced to life imprisonment, after which time their cases do not receive the same level of legal attention afforded those facing execution. But using a medical studies technique known as survival analysis, Gross and three other researchers, including a biostatistics expert, determined that 4.1 percent of U.S. death row inmates were likely innocent. "The rate of erroneous conviction of innocent criminal defendants is often described as not merely unknown but unknowable. We use survival analysis to model this effect, and estimate that if all death-sentenced defendants remained under sentence of death indefinitely at least 4.1 percent would be exonerated," the study's authors University of South Carolina statistics professor John Grego, who did not participate in the research, The study concluded that a "comparatively low" number of innocent defendants have been executed in the United States. "Our data and the experience of practitioners in the field both indicate that the criminal justice system goes to far greater lengths to avoid executing innocent defendants than to prevent them from remaining in prison indefinitely," the study's authors wrote. Just last month, Glenn Ford, a 64-year-old Louisiana man who had spent 30 years on the state's death row after being convicted by an all-white jury for a murder he did not commit, The statistical analysis , published in the peer-reviewed Proceedings of the National Academy of Sciences, concludes that 1 in 25 prisoners on death row is probably innocent. That means it's very likely that at least several of the 1,320 people executed since capital punishment was reinstated in 1977 were innocent, the study's authors claim.From 1973 until 2004, 1.6 percent of prisoners on America's death rows, or 138 individuals, have been exonerated and freed after proving their innocence. Samuel Gross of the University of Michigan Law School, the study's lead author, says the majority of wrongfully convicted and condemned prisoners are never exonerated or freed.Researchers attempting to determine inmate innocence have long been hamstrung by the fact that 60 percent of death row prisoners are eventually removed from death row and sentenced to life imprisonment, after which time their cases do not receive the same level of legal attention afforded those facing execution.But using a medical studies technique known as survival analysis, Gross and three other researchers, including a biostatistics expert, determined that 4.1 percent of U.S. death row inmates were likely innocent."The rate of erroneous conviction of innocent criminal defendants is often described as not merely unknown but unknowable. We use survival analysis to model this effect, and estimate that if all death-sentenced defendants remained under sentence of death indefinitely at least 4.1 percent would be exonerated," the study's authors wrote . "We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States."University of South Carolina statistics professor John Grego, who did not participate in the research, told the Associated Press that it may be better to use the margin of error in the study, placing the true innocence rate at somewhere between 2.8 and 5.2 percent.The study concluded that a "comparatively low" number of innocent defendants have been executed in the United States."Our data and the experience of practitioners in the field both indicate that the criminal justice system goes to far greater lengths to avoid executing innocent defendants than to prevent them from remaining in prison indefinitely," the study's authors wrote.Just last month, Glenn Ford, a 64-year-old Louisiana man who had spent 30 years on the state's death row after being convicted by an all-white jury for a murder he did not commit, walked out of prison a free man following the discovery of suppressed exculpatory evidence. More about Death row, Capital punishment, Executions, innocent execution, Death penalty More news from Death row Capital punishment Executions innocent execution Death penalty