The Detection of Clusters in Rare Diseases

SUMMARY Tests for clustering of rare diseases investigate whether an observed pattern of cases in one or more geographical regions could reasonably have arisen by chance alone, bearing in mind the variation in background population density. In contrast, tests for the detection of clusters are concerned with screening a large region for evidence of individual ‘hot spots’ of disease but without any preconception about their likely locations; the results of such tests may form the basis for subsequent small area investigations, statistical or non‐statistical, but will rarely be an end in themselves. The main intention of the paper is to describe and illustrate a new technique for the identification of small clusters of disease. A secondary purpose is to discuss some common pitfalls in the application of tests of clustering to epidemiological data.