A method for measuring a degree of association (58) between, and for selectively creating a grouping (40) of, n plurality of spatially referenced physical events (22) of a predetermined physical characteristic. The method includes the steps of assembling n plurality of physical events (26), assembling a universe of possible sample locations (36), determining a reference distribution (54), determining a restricted distribution (56), and determining the degree of association (58) between the n plurality of physical events. Specifically, the physical events each have an indicia of location and a physical characteristic above a threshold. The step (54) of determining a reference distribution is conducted by calculating a test statistic (78) for each of n plurality of random allocations (74) of the n plurality of physical events over the selected n plurality of sample locations. Further, the step (58) of determining a restricted distribution includes calculating the test statistic (82) for each of n" plurality of restricted random allocations (80) of the n plurality of physical events over the n plurality of sample locations (62).