determining regions of significance, values of the environmental exposure under which genotypes differ significantly on phenotypic measures. In the instance of discrete environmental measures, this approach is simple since it involves comparing genotypes on phenotypic measure at each level of the environment to determine where they differ significantly. For continuous measures of exposure this can be reformulated as the “pick-a-point” or simple slopes approach, where significant differences in mean or slope can be tested at specific values of the continuous environment (Rogosa, 1980). The obvious weakness of this approach is its reliance on selection of arbitrary environmental values for testing. A preferred approach relies on the calculation of regions of significance or the range of values of the environment where the genotype groups differ significantly on phenotype values. As reviewed in Preacher et al (Preacher, Curran, & Bauer, 2006), this can be accomplished using the Johnson-Neyman (Johnson & Neyman, 1936) approach or the use of confidence bands to identify the range of environmental measures on which the values differ on slope.
