Waller, L.A., Turnbull, B.W., Clark, L.C.,
and Nasca, P. (1994) "Spatial Pattern Analysis to Detect
Rare Disease Clusters" in Case Studies in Biometry,
N. Lange, L. Ryan, L. Billard, D. Brillinger, L. Conquest,
and J. Greenhouse (eds.) New York: John Wiley and Sons.