In total, 156 prisoners have been either acquitted, or received pardons or commutations on the basis of possible innocence, between 1973 to 2015. Death penalty opponents often argue that this statistic shows how perilously close states have come to undertaking wrongful executions; proponents point out that the statistic refers only to those exonerated in law, and that the truly innocent may be a smaller number. Statistics likely understate the actual problem of wrongful convictions because once an execution has occurred there is often insufficient motivation and finance to keep a case open, and it becomes unlikely at that point that the miscarriage of justice will ever be exposed.

Between 1973 and 2015, how amny prisoners were acquitted or received pardons or commutations of their death sentences due to possible innocence?