Last month, a judge in Terre Haute, Indiana released David Scott from prison. Scott was freed after serving more than 20 years for a murder he didn't commit. He was 15 when the crime was committed.

Also last month in Greeley, Colorado, Tim Masters was released after DNA cleared him of a murder for which he had been convicted, and served nearly 10 years. He was also 15 when the crime was committed.

Police pursued him for 12 years before earning his conviction. He's now 36, and has spent most of his life under a cloud of suspicion for a crime he didn't commit.

Scott and Masters are just two of more than 200 people freed by advances in DNA testing after being convicted of crimes they didn't commit.

It's true that 200 don't seem like a large number, particularly when you consider how many cases make their way through U.S. courts each day. But not every criminal case involves biological evidence.

Typically, only murder and rape cases do, and even within those cases, conditions need to be just right. It needs to be a case where the biological evidence available was critical in securing a conviction.

The evidence needs to have been preserved, and still in a condition that's testable. Factor in all of this, and the pool of cases available for DNA testing shrinks considerably, and that 200 plus figure starts to become a significant proportion of the total cases under consideration.

Consider Dallas County, Texas. It has the highest exoneration rate in the country, and is second only to New York and Chicago in total exonerations.

This is largely because of two interesting anomalies. First, because of budget constraints the county began outsourcing its crime lab work to a private contractor in the 1980s. Unlike many jurisdictions, then, where district attorneys destroy case files after a defendant has exhausted his appeals, the private facility where Dallas was sending its lab work to has biological evidence for cases going back 20 years.

At the same time, Dallas' has an (unfortunately) one-of-a-kind district attorney named Craig Watkins, who not only doesn't fight wrongful conviction claims, he is actively working with the Texas Innocence Project to find and clear people doing time for crimes they didn't commit.

Perhaps it's mere coincidence that the same city where, fortuitously, evidence has been preserved going back two decades, and a crusading young district attorney recognizes that there's more to his job than throwing people in prison, would also be the city with the highest exoneration rate in the country.

Perhaps Dallas just has a uniquely flawed criminal justice system, and has now stumbled into an uniquely serendipitous set of circumstances to help uncover it.

Unfortunately, the more likely and troubling explanation is that the rest of the country is just as flawed as Dallas — what is unique about Dallas isn't the number of innocent people in prison there, only that we're finding out about them.

In an illuminating article published last month by the Columbia Law Review, Brandon L. Garret looked at the trials and appeals of 200 people convicted of violent crimes for which they were later exonerated.

Garret's question gets to the crux of the wave of exonerations since the onset of DNA testing: How is it that our criminal justice system, with all of its checks and balances, could allow so many innocent people to be convicted of rape or murder? How did it allow innocent people to be sentenced to death?

Garret found that a variety of factors contributed to the initial wrongful convictions.

False eyewitness testimony was the overwhelming factor (79 percent of the cases), followed by faulty forensic science (55 percent), and false testimony from informants working for the police (18 percent).

In 16 percent of the cases, the defendant actually falsely confessed to the crime. False confessions are common among young and mentally ill suspects, particularly when subjected to harsh interrogation from police.

More troubling was what Garret found in the appeals process.

Many people wrongly assume appeals courts serve as a kind of backup for trial courts, guarding against innocent people slipping through the system. In truth, appeals courts rarely consider the actual guilt or innocence of a criminal defendant.

Most of the time, they address procedural matters relating to how the trial was administered, whether the judge issued appropriate rulings regarding evidence and witnesses, and whether the state properly protected the defendant's constitutional rights.

Garret found that of the 200 people convicted for crimes for which they were later exonerated, just eighteen were granted reversals by the appellate courts.

Of the rest, 67 had their appeals denied with no written ruling at all. In 63 cases, the appellate court's opinion referred to the defendant's guilt. In 12 other cases, it referred to the "overwhelming" evidence of guilt.

In the remaining cases, the appeals courts either found the defendant's appeal without merit, or found some merit in his claims, but found that the trial court's errors were "harmless," or unlikely to have affected the jury's verdict.

Keep in mind, these are all cases in which the defendant was later determined to be actually innocent of the crime for which he was convicted. More alarmingly, Garret found in his research of these 200 cases that "even after DNA testing became available, courts and law enforcement also posed obstacles to conducting DNA testing, and then denied relief even after DNA proved innocence."

Many were convicted despite DNA testing pointing to their innocence, and 41 had to rely on the mercy of a governor's pardon power because, despite their proven innocence, they had already exhausted their appeals, and could make no further claims in court.

"Thus for some," Garret concludes. "Even once DNA evidence exonerated them, our judicial system was unwilling or unable to provide a remedy."

Garret's study is chilling.

Even if these 200 cases represent a small percentage of the subset of cases for which DNA testing can conclusively point to a defendants guilt or innocence — say one or two percent — it's safe to assume that the flaws in the criminal justice system that allowed them to happen exist in all criminal cases, not just rape or murder cases.

The same overeager prosecutors, corrupt or incompetent forensics experts and cops, mistaken eyewitnesses, and indifferent courts that prosecute and oversee these cases also move thousands of cases through the system for which there's no safety net of DNA testing.

If it's this difficult for an innocent person to clear his name in cases where there's science available to deliver a definitive answer, imagine the people now wrongly sitting in a jail cell for drug offenses, theft, or for violent crimes for which there was no available biological evidence—people for whom science offers little hope for relief.