Agency for Toxic Substances and Disease Registry Case Studies in Environmental Medicine (CSEM)
Disease Clusters: An Overview
Define the Population Denominator
Once an acceptable case definition exists and the reported cases have been confirmed, the public health department will determine whether the suspected cluster is actually "unusual." This determination calls for an explicit comparison between the number of cases actually observed and the number that would have been expected under normal conditions. The numerator cases (e.g., four teachers with breast cancers) must ultimately be compared to some denominator population (e.g., expected number of teachers diagnosed with breast cancer in such an elementary school population). To make such comparisons, it is necessary to first define the population at risk and then determine the number of cases normally expected in that population. The public health epidemiologist undertakes the task of defining the population at risk.
All clusters appear unusual when first discerned; yet most are due to chance. Accordingly, it is necessary to restrain the natural tendency to jump to causal conclusions. Among the most important initial concerns are considerations of biologic plausibility. One aspect of such considerations is to thoughtfully define the nature of the cluster; another is to determine the scientific support for the obvious hypotheses.
