ABSTRACT
Imagine yourself in the role of a public health official in an urban area. You are asked to monitor data on admissions to emergency departments for respiratory distress and to report on any abnormal increases that may occur. As part of your response, you decide to both tabulate and map the daily, weekly, and monthly reports of such occurrences. As you examine your first map depicting the location of new cases, how do you decide whether the map of occurrences has an interesting pattern? There may be apparent clusters of incidents, but these may simply reflect geographic patterns of population density. Perhaps you have accounted for population density and also for the age structure of the population. Is there still a pattern on the map that deviates significantly from some simple random assignment of cases to the population? The geographic pattern may reflect other factors-perhaps individuals living near hospitals are more likely to be admitted for respiratory distress.
