Median sensitivity and specificity for varied areal unit population sizes and cluster relative risks. (Legend applies to all subplots.). A circular scan and elliptical scan were used to find the most likely cluster for each simulation. Two basic cluster shapes were used: circle and a rectangle with varying aspect ratio from 2 to 10. The cluster encompassed approximately five percent of the locations with a risk ratio of 1.5, 2.5 or 5.0. The study area was partitioned into areal units of maximum populations using the Epiunit method. Partitioning sets used had maximums of 500, 1,000, 2,000 or 4,000 locations per unit. Each scenario was simulated 100 times with a background prevalence of 0.004 and the location distribution of RC A as background.