New survey suggests more than 20,000 innocent people are behind bars in the US.

Before we talk about how many people may be behind bars for crimes they did not commit, we must acknowledge that it's nearly impossible to know—only broad estimates are possible. There are several key reasons, experts say, why a number is so hard to ascertain. Because the sprawling criminal justice system is a patchwork of federal, state, county, and municipal courts, prisons, and jails—each with its own system (or lack thereof) of record-keeping and data-reporting—we don't even know how many people are convicted, let alone wrongfully convicted, of crimes in the United States. "We don’t even have a denominator," says University of Virginia law professor Brandon Garrett. "But the wrongful convictions we do know about suggest that there's a big problem."

Extrapolating from the 281 known DNA exonerations in the US since the late 1980s, a conservative estimate is that 1 percent of the US prison population, approximately 20,000 people, are falsely convicted.

Moreover, a tiny amount of total criminal cases actually go to trial. Nineteen out of twenty, or 95 percent, of convictions in the US are by plea bargain—and so we know little about them. They "generate virtually no records that can be retrieved," writes Gross: "no trial transcripts, no appeals, frequently no court hearings of any sort, in many cases no description of the investigation at all beyond a single police report, which (if it could be found) might include little factual information of any value."
http://motherjones.com/politics/2011/12/innocent-people-us-prisons