Spatial analytical methods and geographic information systems: use in health research and epidemiology.

Person, place, time: these are the basic elements of outbreak investigations and epidemiology. Historically, however, the focus in epidemiologic research has been on person and time, with little regard for the implications of place or space even though disease mapping has been done for over a hundred years. The development of geographic information systems (GISs) over the last 20 years has provided a more powerful and rapid ability to examine spatial patterns and processes. This, in turn, has fostered the discussion of such policyrelevant issues as health services and planning (1), as well as the use of GISs for epidemiologic investigations and disease surveillance. Methods of spatial analysis are given little, if any, introduction in modern epidemiology texts, and few epidemiologists have ventured further than making dot or choropleth maps, or listing geographic units along with rates of disease in tabular form (2-6). Spatial patterns are frequently intricate and complex, while most spatial methods used by epidemiologists can capture or identify only gross, simplistic patterns (2). Epidemiologists understand that disease processes have an historical (time) component, and formal methods of time series and hazard analyses are well-developed to study them. Few, however, recognize that every epidemic also has a geography (space). Some epidemiologists may not be aware that evaluation of the spatial distribution of measures of disease risk may provide etiologic insight. The logic of using geography to study disease or health care is derived from appreciation of factors causing non-uniformity of disease distribution (2). These fac-