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."
Read more: http://motherjones.com/politics/2011/12/innocent-people-us-prisons