Statistical issues in Serial Killer Nurse cases

We discuss statistical issues in cases of serial killer nurses, focussing on the Dutch case of the nurse Lucia de Berk, arrested under suspicion of murder in 2001, convicted to life imprisonment, but declared innocent in 2010; and the case of the English nurse Ben Geen, arrested in 2004, also given a life sentence. At the trial of Ben Geen, a statistical expert was refused permission to present evidence on statistical biases concerning the way suspicious cases were identified by a hospital team of investigators. The judge ruled that the expert's written evidence was merely common sense. An application to the CCRC to review the case was turned down, since the application only presented statistical evidence but did not re-address the medical evidence presented at the original trials. This rejection has been successfully challenged in court, and the CCRC has withdrawn it. The paper includes some striking new statistical findings on the Ben Geen case as well as giving advice to statisticians involved in future cases, which are not infrequent. Statisticians need to be warned of the pitfalls which await them.