PERCEPTION OF CLUSTERS IN STATISTICAL MAPS

Two experiments observed performance on a cluster identification task across a variety of common statistical maps. Stimulus maps displayed m ortality rates for several diseases and subjects had to identify regio ns of the map that were perceived to form a cluster of particularly hi gh (or low) mortality. Subjects marked the perceived centroid of each cluster, and analyses focused on the dispersion of centroid location a cross subjects. Under these circumstances, monochrome classed chorople th maps were found to minimize dispersion, compared to a two opposing colours scheme, a dot density map, a pie map, and a categorical (hue-b ased) colour scheme. Maps using a familiar geographical unit (i.e. a U .S. state) supported better recall of the information than maps using less familiar and smaller geographical units. The results were found t o be interpretable within current cognitive theory.