Effect size: A quantitative measure of the magnitude of a genetic association, determined by its contribution to the total genetic variance of the trait as well as the statistical power to detect it in an association study. Effect sizes are also influenced by gene-by-environment interactions, which may result in population-specific variability when a specific environmental exposure is more common, increasing the apparent strength of a genetic association. For most types of effect size with a center around zero (for example, a beta-coefficient), a larger absolute value always indicates a stronger effect; the main exception is if the effect size is an odds ratio (which centers around one).
