Residual effects are also often correlated when we have repeated measures on the same subjects (e.g., measuring the same trees or animals at different ages) or when we measure multiple traits on each experimental unit and want to fit multivariate models. A trait measured at different ages can be treated in some cases as just a specific form of a multivariate model with different traits. For example tree height could measured at ages 2, 4 and 6 and recorded as Heigh2, Heigh4, Heigh6. The algebraic expression of residual variance matrix for repeated measure and for multivariate models is