Statistical Analyses in the case of an Italian nurse accused of murdering patients

TL;DR

This paper discusses the statistical challenges in distinguishing genuine criminal activity from coincidental event clusters in hospital settings, highlighting how apparent associations can be misleading due to underlying patterns of patient deaths and nurse shifts.

Contribution

It demonstrates that observed correlations between nurse presence and patient deaths can be spurious, emphasizing the importance of considering hospital operational patterns in statistical analyses.

Findings

Most deaths occur in the morning when many nurses are on duty.

Higher death rates coincide with nurse presence due to shift patterns, not necessarily causation.

Statistical analysis must account for shift schedules to avoid false associations.

Abstract

Suspicions about medical murder sometimes arise due to a surprising or unexpected series of events, such as an apparently unusual number of deaths among patients under the care of a particular nurse. But also a single disturbing event might trigger suspicion about a particular nurse, and this might then lead to investigation of events which happened when she was thought to be present. In either case, there is a statistical challenge of distinguishing event clusters that arise from criminal acts from those that arise coincidentally from other causes. We show that an apparently striking association between a nurse's presence and a high rate of deaths in a hospital ward can easily be completely spurious. In short: in a medium-care hospital ward where many patients are suffering terminal illnesses, and deaths are frequent, most deaths occur in the morning. Most nurses are on duty in the morning, too. There are less deaths in the afternoon, and even less at night; correspondingly, less nurses are on duty in the afternoon, even less during the night. Consequently, a full time nurse works the most hours when the most deaths occur. The death rate is higher when she is present than when she is absent.