4.1.1 Cluster Studies
A cancer cluster is an aggregation of a relatively unexpected high number of cases. Clustering can be “spatial,” when the disease in question has a higher incidence rate in some places than in others, or “temporal,” when the incidence rate is higher at a specific time compared to other times. A disease cluster can also be “spatiotemporal.” Testing involves comparing the observed number of cases with the number expected, based on the size and age composition of the population.
The scientific reason to examine disease clusters is to learn about the causes of the cluster and, by extension, gain insight toward the causes of disease. Epidemiologists and public health workers recognize the value of historic examples of cancer cluster examination which contributed to the recognition of human carcinogens in those situations. Typically, exposure was high, prolonged, and well defined. In contrast, most cluster reports involve exposures that are low and poorly defined, and the cases involved are a mix of unrelated, relatively common cancers. For these reasons there is skepticism regarding the scientific value of the investigation of reported clusters (Neutra, 1990; Rothman, 1990).
In a rather provocative summary of the reasons why—with a few exceptions—there is little scientific or public health purpose to investigate individual disease clusters, Rothman (1990) explains that the boundaries of the space and time that encompass the cluster should be clearly defined before examination of the cluster and should not be defined after the fact to capture a population that has experienced the high disease rate. This interpretation has been described as the “Texas sharpshooter’s” procedure in which the shooter first fires his shots randomly at the side of the barn and then draws a bull’s eye around each of the bullet holes. This kind of process tends to produce clusters of causally unrelated cases of no etiologic interest. As noted by Rothman (1990), assigning statistical significance to a reported cluster requires clear definitions of the populations, regions, and/ or time periods under consideration, often a challenging undertaking.
4.1.2 Ecologic Studies
An ecologic study (sometimes referred to as a geographic study or correlation study) evaluates the relationship between an exposure and a disease in some aggregate group of individuals, but not specific individuals, such as those living in a country, a county, a community, or a neighborhood. This is in contrast to case-control and cohort studies where the unit of analysis is the individual. In an ecologic study, average measures of exposure and disease frequency are obtained for each aggregate, and the analyses focus on determining whether or not the aggregates with high levels of exposure also display high disease rates. For example, in a study that uses counties as the