The Centers for Disease Control and Prevention (CDC) advocate a multi-step approach for investigating disease clusters (1990). ClusterSeer offers tools for the cluster assessment stage, steps 2a and 2c.
Initial contact and response. An agency is notified of a perceived cluster and decides whether further evaluation is necessary.
Cluster assessment.
Preliminary evaluation. This step provides a rough estimate of the probability of the perceived cluster occurring by chance. In this step, determine the geographic area and time to examine and find a reference population for comparison. Then, calculate statistics for the perceived cluster and compare them to the reference population.
Case evaluation. Verify the case reports are accurate.
Occurrence evaluation. A more thorough descriptive evaluation, repeating the preliminary evaluation with verified data. This step also includes a literature review to investigate an association between the cluster and exposure or source.
Major feasibility study. Here, a case-control study is designed and any environmental monitoring scheme planned.
Etiologic investigation. This step implements the study planned in Step 3. It evaluates the link between the hypothesized cause of the cluster and the disease. It does not necessarily give information on the causes of the original cluster, but evaluates plausible causes.
Most studies of apparent disease clusters are not substantiated after early data exploration. Most end at stage 2, after finding no significant clustering. For example, The Minnesota Department of Health received 420 reports of apparent clusters between 1981-8 (Bender et al. 1990). About 95% of these investigations were ended at stage 2, with no clustering found. Of the remaining 5%, only 1/5, or 1% of the original total, warranted an epidemiological study. A similarly low rate of cluster verification occurred in a study of 61 cluster investigations between 1978-84 at the National Institute for Occupational Safety and Health (Schulte et al. 1987). Most apparent clusters did not have a greater than expected number of cases, and of those that did, most could not be explained by occupational exposure.