Statistics and the Law: the case of the nonchalant nurse

Willem van Zwet
University of Leiden


        In a hospital in The Hague a number of unexpected cases of death or re-animation of patients occurred. When it was found that in all of these cases a nurse named Lucia was present, she was arrested and tried for multiple murder. The case for the prosecution rested on statements of witnesses, toxicological evidence and a statistical analysis showing that her presence in all cases could not be attributed to chance. On the basis of this Lucia was sentenced to life imprisonment.

        The case was appealed by the defendant and considered by the Appellate Court in The Hague. The defense now produced another expert witness who claimed that the statistical evidence presented earlier was unconvincing. In a newspaper interview a third expert went quite a bit farther and claimed that the statistical analysis was completely wrong and when performed correctly, should have led to a verdict of not guilty. In the resulting confusion the Appellate Court made it clear that it could not credit any of the statistical arguments and confirmed the life sentence without mentioning the word statistics at all, though clearly still impressed by the unlikely presence of the defendant in all cases. The Supreme Court later upheld the life sentence.

        In this talk I will explain that the statistical experts failed to make the main issues clear to the judges, that a legal education does not prepare prosecutors and penal judges to understand these issues, and that the competence of the police in such matters is at best dubious. After much public protest, a government commission is presently considering a request to re-open  the case.