This is Radley Balko‘s question to a very important question. And is the title to a very well written article by him, posted at Reason.com.
This concerns wrongly convicted, innocent, people incarcerated for serious crimes. As of now, there have been at least 250 persons exonerated by DNA testing. Since 1989.
Could be more. It is very difficult to determine how many and how to determine.
Read the article.