Introduction

Abstract

In epidemiological studies, it is often of importance to evaluate whether a disease is randomly distributed or tends to occur as clusters over time and/or space after adjusting for known confounding factors, that may provide clues to the etiology of the disease. There has recently been great public concern about clustering of health events such as the occurrence of childhood leukemia, birth defects, and cancer. For example, since the early 1960s, it has been argued that childhood leukemia could be caused by either an infectious agent or an environmental toxin. Therefore, many researchers have examined space-time clustering of childhood leukemias in relation to the date and place of onset or diagnosis using various methods, including the Knox test. Furthermore, since the 1980s, there has been growing interest in the relation between the risk of a disease and proximity of residence to a prespecified putative source of hazard. It is well-known that the apparent excess of cases of childhood leukemia near a nuclear reprocessing plant such as that in the village near Seascale facility at Sellafield has been extensively investigated (for example, see Bithell et al., 1994). More recently, there has been great public concern about the health effects of dioxins, organic compounds such as polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs), emitted from municipal solid waste incinerators (for example, see Elliott et al., 1996). In 1990, the Centers for Disease Control and Prevention (1990a, 1990b) issued the “Guidelines for investigating clusters of health events”. In its appendix (1990b), a “summary of methods for statistically assessing clusters of health events” is provided as a resource for investigators who may become involved with the statistical aspect of reported clusters of health events.