Little, James K. (1985 ASQC and the American Statistical Association) U.S. Department of Agriculture, Madison, WI
A quadratic form found in the Andrews-Pregibon (AP) statistic may be decomposed into two terms. One of the terms has a simple geometric interpretation that relates naturally to the expression of influence found in Cook's statistic. The measure of influence derived from the quadratic form is easy to compute once the two factors of the AP statistic have been found. This measure suggests a computing strategy for diagnostics and leads to an exact relation between the AP statistic and Cook's statistic.