This is a difficult question because it's hard to find an accurate denominator for comparison.
Nobody thinks every innocent person has been identified through DNA testing, and indeed no biological evidence exists to test in the vast majority of criminal cases. So even though we know 39 Texans have been exonerated by DNA, we don't know what percentage of criminal convictions overall are false.
One of the few datasets that generates a statistically viable denominator comes from capital murder cases, for which a new study from Michigan State provides a new, national calculation:
Among defendants sentenced to death in the United States since 1973, at least 2.3 percent—and possibly more—were falsely convicted, said U-M law professor Samuel Gross in a study co-authored by Barbara O'Brien, a professor at Michigan State University College of Law.The exoneration rate in Texas for capital murder convictions is slightly lower than in this national study.
If defendants who were sentenced to prison had been freed because of innocence at the same rate as those who were sentenced to death, there would have been nearly 87,000 non-death row exonerations in the United States from 1989 through 2003, rather than the 266 that were reported, the study said.
"The main thing we can safely conclude from exonerations of falsely convicted defendants is that there are many other false convictions that we have not discovered," said Gross, whose research has focused on the death penalty, false convictions and eyewitness identification.
Since 1989, nearly all exonerations in the United States fall into three categories: rape convictions, because of post-conviction DNA testing; murder convictions, and especially death sentences, which are subjected to much more detailed post-conviction reinvestigation than other convictions; and a few groups of false drug and gun possession convictions that were produced by concerted programs of police perjury that later unraveled.
As result, researchers know little about false convictions among crimes of violence other than murder or rape, even though false convictions for robbery could greatly outnumber those for rape and murder. And researchers know next to nothing about false convictions for other types of crimes, such as property crimes, misdemeanors and white collar crimes.
Another dataset that lends itself to statistically valid innocence estimates come from DNA exonerations. In Texas, 3.3% of cases solved by DNA evidence resulted in exonerating convicted defendants.
So let's guess that the false conviction rate in Texas is somewhere between 2.3-3.3%: With around 155,000 prisoners, that would mean between 3,500 and 5,000 or so current Texas prison inmates were falsely convicted.
Another 10-15,000 falsely convicted people are on the probation rolls, this data implies - perhaps even more since innocent people may be more likely to accept a plea for probation than risk incarceration for something they didn't do.
That's a helluva lot of folks.