Estimating how many death row inmates are wrongfully convicted or executed has proven exceedingly difficult, as cases are rarely overturned.
But a new analysis technique -- borrowed from long-term medical treatment studies -- suggests that for every exonerated death row prisoner there is another wrongfully convicted inmate who doesn't make it out alive.
As study author Samuel Gross, a law professor at the University of Michigan Law School, explained to Scientific American, the new analysis technique allows researchers to "actually come up with a valid estimate of the rate of false convictions -- knowing something that people say [in criminal justice] is not knowable."
The statistical method is usually used to determine how a specific treatment affects survival rates. In this case, the illness was the threat of execution and the treatment was the removal of that threat -- being re-sentenced to life in prison. The statisticians determined that the longer someone is on death row, the more likely they are to be exonerated.
In applying the new analysis to death sentence stats from 1973 to 2004, researchers estimated that 340 people were wrongfully sentenced to death. That's more than double the number of death row prisoners, 144, who were exonerated during that time. Put another way, for those 31 years, 4.1 percent of death row inmates were placed there wrongly.
The researchers suggest that those rates likely remain the same, as DNA evidence rarely plays a significant role in murder convictions.
"There are no other reliable estimates of the rate of false conviction in any context,” the study's authors wrote. The research was published online this week in the Proceedings of the National Academy of Sciences.