TESTING FOR REGIONAL DIFFERENCES IN MEANS - DISTINGUISHING INHERENT FROM SPURIOUS SPATIAL AUTOCORRELATION BY RESTRICTED RANDOMIZATION

Tests for differences among regional means are typically carried out b y analysis Of variance (ANOVA). When such data are spatially autocorre lated (SA), the assumptions of ANOVA are not met, giving rise to exces sive type I error rates. Two spatially adjusted ANOVA methods, Griffit h's and COCOPAN, have been proposed to overcome this problem. In this study we show, by means of extensive simulations, the magnitude of the error rates introduced by SA induced in isolation-by-distance models typical of those used in population genetics. For data suspected of ex hibiting such SA, we propose a strategy for distinguishing between inh erent SA, generated within the data by a contagious process, and spuri ous SA, introduced by regional differences in means. The approach adop ted is that of restricted randomization Of distance matrices. We also furnish error rates and power estimates for both Griffith's method and COCOPAN. In addition to the simulated data, the methods are applied t o an actual example from plant population biology.