A WSJ Law Blog summary covers this study from "innocence" projects. Exonerations are being tracked in an online registry:  exonerationregistry.org. The big picture questions and outcome? They are: 

"It raises the question: Just how many exonerations (due to DNA evidence or otherwise) happen in the U.S.?

The answer, it turns out, is over 2,000 in the last 23 years, according to a study out Monday from the University of Michigan Law School and the Center on Wrongful Convictions at Northwestern University."