| Sumario: | The writers present a method for identifying local patterns of spatial association that computes exactly the p-values of the local Moran's Ii, local Geary's ci, and the modified Ord and Getis G statistics based on the distributional theory of quadratic forms in normal variables. An approximate method, three-moment x2approximation, with explicit calculation formulae is also proposed, in order to achieve a computational cost lower than the exact method. Numerical evaluation on the accuracy of the approximate null distributions of the local statistics reveals that although the proposed three-moment x2 method is useful in some situations, it is inappropriate for approximating the null distribution of I.
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