Notes on the Implications of Ignoring Bayes’ Rule in Search and Rescue Practice in the UK.

Thomas Bayes (1702-1761) has had a big influence on the science of inference since he discovered the mathematically correct way of adjusting probabilities to account for new evidence. Nonetheless, it is still the case that in practice it is not always clear where and when to apply the rule he derived, or the consequences of not doing so. In this note, the effects of not doing so when searching an area of ground for a missing person (misper), where the chances of finding them depends both on whether they are there and how well the ground is searched, is investigated. This investigation suggests that within the range of probabilities that generally apply to search operations in rural settings in the UK, the widespread failure to apply Bayes’ rule may incline search managers to widen search areas more than is warranted by the evidence and may thereby reduce overall search effectiveness (ceteris paribus).