The Saddlepoint Approximation of Moran’s I’s and Local Moran’s Ii’s Reference Distributions and Their Numerical Evaluation

Global Moran’s I and local Moran’s Ii are the most commonly used test statistics for spatial autocorrelation in univariate map patterns or in regression residuals. They belong to the general class of ratios of quadratic forms for whom a whole array of approximation techniques has been proposed in the statistical literature, such as the prominent saddlepoint approximation by Offer Lieberman (1994). The saddlepoint approximation outperforms other approximation methods with respect to its accuracy and computational costs. In addition, only the saddlepoint approximation is capable of handling, in analytical terms, reference distributions of Moran’s I that are subject to significant underlying spatial processes. The accuracy and computational benefits of the saddlepoint approximation are demonstrated for a set of local Moran’s Ii statistics under either the assumption of global spatial independence or subject to an underlying global spatial process. Local Moran’s Ii is known to have an excessive kurtosis and thus void the use of the simple approximation methods of its reference distribution. The results demonstrate how well the saddlepoint approximation fits the reference distribution of local Moran’s Ii. Furthermore, for local Moran’s Ii under the assumption of global spatial independence several algebraic simplifications lead to substantial gains in numerical efficiency. This makes it possible to evaluate local Moran’s Ii’s significance in large spatial tessellations.