The proportion of death row inmates executed to those set free isn’t exactly encouraging. Since 1972, 1,500 people have been executed in the United States. Over that same time, “166 former death-row prisoners have been exonerated of all charges and set free,” according to the Death Penalty Information Center.

Extrapolating from the cases in which death row inmates were proven to have not committed the crimes of which they were convicted, a 2014 study estimated that 4.1 percent of all death row inmates could be exonerated. “We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States,” the authors added.

That’s an awful lot of people cooling their heels behind bars for crimes they didn’t commit.