Updated graphs of investigation and prosecution contribution factors in nearly 1600 exonerations proved by the National Registry of Exonerations. (wait a bit for download, scroll down for other stats).
Look for GREEN: False or Misleading Forensic Evidence 23% 363/1597
Look for ORANGE: False Confession 13% 202/1597
Look for Red: Perjury or False Accusation 56% 888/1597
Look for Purple: Official Misconduct 46% 727/1597
Look for Blue: Mistaken Witness ID 34% 535/1597
For a contrasting opinion, you can read this.