A miscarriage of justice, also known as a wrongful conviction, occurs when a person is convicted and punished for a crime that they did not commit. False confessions by psychologically vulnerable defendants are one of the main causes of wrongful convictions, sometimes exacerbated by coercive police interrogations. Tampering with evidence and other police misconduct also contribute to miscarriages of justice. The Innocence Project keeps statistics on cases where convicted defendants have subsequently been exonerated, usually by advances in the science of DNA. In many instances, wrongful convictions have not been overturned for several decades - sometimes after the innocent person has been executed, released from custody, or has died. The true prevalence of miscarriages of justice is hard to measure because many wrongful convictions are never overturned.