Freddie Peacock of Rochester, New York, was convicted of rape in 1976. This year he became the 250th person to be exonerated by DNA testing since the technique was first used in 1989. According to a new report by the Innocence Project, those 250 prisoners served a total of 3,160 years; 17 spent time on death row. Remarkably, 67 percent of them were convicted after 2000, a decade after the onset of modern DNA testing. The glaring question: How many more are there?

Calculating the percentage of innocents now in prison is a tricky and controversial thing to do. The certainty of DNA testing means we can be positive the 250 defendants listed in the Innocence Project report didn't commit the crimes for which they were convicted. But there are hundreds of other cases in which convictions have been overturned due to a lack of evidence, recantation of eyewitness testimony, or police or prosecutorial misconduct, and where no DNA evidence exists to definitively establish guilt or innocence. Those were wrongful convictions because there wasn't sufficient evidence to overcome reasonable doubt, but we can't be sure all of the accused were actually innocent.

Most prosecutors fight requests for post–conviction DNA testing. That means the discovery of wrongful convictions is limited by the time and resources available to the Innocence Project and similar legal aid organizations who fight for tests in court. It's notable that in one of the few jurisdictions where the district attorney is actively seeking out wrongful convictions—Dallas County, Texas—the county by itself has seen more exonerations than all but a handful of individual states. If prosecutors in other jurisdictions were to follow Dallas County District Attorney Craig Watkins' lead, that 250 number would be significantly higher.

Still, some officials aren't impressed. One of the more farcical attempts to underplay the growing number of DNA exonerations came in a concurring opinion that Supreme Court Justice Antonin Scalia wrote in the 2005 case Kansas v. Marsh. Scalia began by dismissing the idea that an innocent person may have ever been executed in America, explaining that if such a tragedy had occurred, "we would not have to hunt for it; the innocent's name would be shouted from the rooftops by the abolition lobby."

Since then the abolition lobby has been shouting about Cameron Todd Willingham, the probably innocent man who was executed in Texas in 2004. But Scalia's argument also betrays a lack of familiarity with how anti–death penalty groups work. While the justice is right that proof of an executed innocent would be good rhetorical fodder for the abolitionists, legal aid groups aren't about to waste their limited resources hunting down mistaken executions when there are living, breathing innocents still to be discovered. Moreover, in many jurisdictions, prosecutors destroy case files after an execution, making any post-execution investigation rather difficult.

Scalia also cited some math from Josh Marquis, an Oregon prosecutor who has held various executive positions at the National District Attorneys Association. Marquis crunched his numbers at a time when there had been about 200 DNA exonerations. He arbitrarily multiplied that number by 10 to come up with 2,000 wrongful convictions. Marquis then took every single felony conviction over the previous 15 years as his denominator and came up with a meager .027 percent wrongful conviction rate. Move along, America. Nothing to see here. Your criminal justice system is performing just fine.

But that figure is ridiculous. First, the subset of cases in which DNA testing can prove (or disprove) guilt is exceedingly small. It is generally limited to most rape and some murder cases. You can throw out the entire body of drug charges and nearly all burglaries, robberies, assaults, and other felonies. As University of Michigan law professor Samuel L. Gross wrote in the Annual Review of Law and Science, "By this logic, we could estimate the proportion of baseball players who've used steroids by dividing the number of major league players who've been caught by the total of all baseball players at all levels: major league, minor league, semipro, college and Little League—and maybe throwing in football and basketball players as well."

If the aim is to calculate the percentage of people who claim they're innocent and who actually are, you might throw out all cases decided by a guilty plea too. But this can get tricky. According to the Innocence Project, more than a quarter of DNA exonerations included a false confession or guilty plea. The plea bargaining process also induces innocent people to plead guilty to lesser crimes to avoid charges that carry longer prison terms.

The Innocence Project cites a study by the Seton Hall law professor D. Michael Risinger that puts the share of innocents in prison at 3 percent to 5 percent. But that study looked only at capital crimes, and it's a much-debated question whether you can extrapolate data from death penalty cases and make assumptions about conviction rates in lesser crime categories. (Those who argue that non-death penalty conviction rates would be higher note that there's more pressure on prosecutors and jurors to hold someone accountable in murder cases. Then again, defendants tend to have better representation in capital cases.) But even dropping below the study's floor, using the 2008 prison population, a 2 percent wrongful conviction rate would mean about 46,000 people are incarcerated for crimes they didn't commit.

Whatever the percentage, DNA testing has exposed some glaring flaws in the system, calling into question traditional assumptions about the value of eyewitness testimony, forensic evidence, confessions, and the appeals process. (In several cases where a defendant was later exonerated by DNA testing, appeals courts not only upheld convictions but noted "overwhelming evidence" of the defendants' guilt.) Since we now know because of DNA testing that misconduct by police and prosecutors produced wrongful convictions in many high-profile murder cases, it is probably safe to assume that similar problems have led to the wrongful conviction of routine drug defendants. The difference is that there is no test to clear those people's names.

Scalia wrote that an exoneration "demonstrates not the failure of the system but its success." But these 250 DNA tests aren't proof that the system is working. They're a wake-up call telling us that it isn't. Instead of falling back on groups like the Innocence Project to serve as unofficial checks against wrongful convictions, cops, prosecutors, judges, and lawmakers should be thinking about why there's so much work for these organizations to do.

Radley Balko (rbalko@reason.com) is a senior editor at reason.