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The word cluster is frequently used in epidemiologic studies of multiple sclerosis (MS), and various forms of cluster studies are potentially powerful tools in the search for clues to the etiology of a disease. The basis for all epidemiologic studies is deviations from a completely random distribution of the disease, and any departure from the random distribution of cases either in space, in time, or along any other axis might be defined as a cluster of cases. A review of the literature shows that such a loose definition of a cluster study has been used. Sixty-one of the 1,181 articles listed in Medline during the period 1966 to 1996 under multiple sclerosis and epidemiology were listed with the additional key word cluster or clustering.
We will restrict this discussion to two types of analysis, which are most frequently referred to as cluster analysis. The most common cluster studies are those attempting to interpret an apparent excess of cases within a small geographic area. This is often referred to as post hoc cluster analysis since such clusters in general are not defined by statistical analysis, but are brought to the attention of the epidemiologist or neurologist by local physicians, patients, the mass media, or others. A second type of cluster analysis is the space-time cluster analysis. This is used to test an infectious etiology more formally.
Post hoc cluster analysis. The intuitive definition of a cluster of cases is an excess number of new cases of the disease within a small area and over a short period. The rationale for studying such a cluster of cases is that if the outbreak of the disease is caused by an environmental factor, one might assume that the population in such an area is more exposed to the putative agent than the general …
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