The rate of erroneous conviction of innocent criminal defendants is often described as not merely unknown but unknowable. There is no systematic method to determine the accuracy of a criminal conviction; if there were, these errors would not occur in the first place. As a result, very few false convictions are ever discovered, and those that are discovered are not representative of the group as a whole. In the United States, however, a high proportion of false convictions that do come to light and produce exonerations are concentrated among the tiny minority of cases in which defendants are sentenced to death. This makes it possible to use data on death row exonerations to estimate the overall rate of false conviction among death sentences. The high rate of exoneration among death-sentenced defendants appears to be driven by the threat of execution, but most death-sentenced defendants are removed from death row and resentenced to life imprisonment, after which the likelihood of exoneration drops sharply. 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% would be exonerated. We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States.AP reports that, nationally, “From 1973 to 2004, 1.6 percent of those sentenced to death in the U.S. — 138 prisoners — were exonerated and released because of innocence.” This study attempts to extrapolate using statistical analysis how many other actually innocent defendants had their death sentence eliminated but were never formally exonerated, boosting the total to 4.1 percent. Noted AP, "The difficulty in identifying innocent inmates stems from the fact that more than 60 percent of prisoners in death penalty cases ultimately are removed from death row and resentenced to life imprisonment. Once that happens, their cases no longer receive the exhaustive reviews that the legal system provides for those on death row."
From what I know about rates of wrongful convictions, that doesn't sound unreasonable to me. I personally believe Texas has executed one or more innocent people over the last three decades, and numerous others have been exonerated after spending time on death row. Others, like Clarence Brandley and Kerry Max Cook, were freed from death row but never formerly exonerated. Indeed, it may be that high-profile death penalty cases have higher false conviction rates than others because of the intense pressure on prosecutors and judges to convict, though there are also plenty of false convictions in lesser cases. Either way, it's almost certain that false convictions in death penalty cases are more likely to be discovered after the fact because of the rigorous examination they receive during the habeas corpus process that almost never occurs for cases that end in a "mere" prison sentence.
That said, many death penalty abolitionists believe that these sorts of analyses may sway the public to oppose capital punishment, a position that simply isn't supported by the data. Among people who believe an innocent person has already been executed, a solid majority support capital punishment. Identifying people wrongfully sent to death row helps impress upon the public the import of rectifying flaws in the justice system that contribute to false convictions. But IMO, especially in Texas, it won't be any sort of silver bullet that convinces people to oppose the death penalty.