Tango's statistic to perform a general clustering test is expressed as follows:
T = (r-p)' A (r-p)
where r' = [O_1/O_+, ..., O_n/O_+], p' = [E_1/E_+, ..., E_n/E_+] and A is a matrix of closeness which measures the cloneness between two zones (the higher the closer).
Tango proposes to take A_ij=exp(-D/phi), where D_ij is the distance between centroids of regions i and j, and phi is a constant that measures how strong is the relationship between regions in a general way.
Tango, Toshiro (1995). A Class of Tests for Detecting 'General' and 'Focused' Clustering of Rare Diseases. Statistics in Medicine 14, 2323-2334.
DCluster, tango.stat, tango.boot, tango.pboot