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7 Command file: Specifying variance structures 1077.2 Variance model specification in ASRemlThe variance models are specified in the AS-Reml command file after the model line, asshown in the code box. In this case just onevariance model is specified (for replicates, seemodel 2b below for details). predict andtabulate lines may appear after the modelline and before the first variance structureline. These are described in Chapter 10.NIN Alliance Trial 1989variety !A.column 11nin89.asd !skip 1yield ∼ mu variety !r repl0 0 1repl 1repl 0 IDV 0.1Table 7.3 presents the full range of variance models available in ASReml. Theidentifiers for specifying the individual variance models in the command file aredescribed in Section 7.5 under Specifying variance models in ASReml. Many of themodels are correlation models. However, these are generalized to homogeneousvariance models by appending V to the base identifier. They are generalized toheterogeneous variance models by appending H to the base identifier.7.3 A sequence of structures for the NIN dataEight variance structures of increasing complexity are now considered for the NINfield trial data (see Chapter 3 for an introduction to these data). This is to givea feel for variance modelling in ASReml and some of the models that are possible.See Section 2.1Before proceeding, it is useful to link this section to the algebra of Chapter 2. Inthis case the mixed linear model isy = Xτ + Zu + ewhere y is the vector of yield data, τ is a vector of fixed variety effects but wouldalso include fixed replicate effects in a simple RCB analysis and might also includefixed missing value effects when spatial models are considered, u ∼ N(0, G) isa vector of random effects (for example, random replicate effects) and the errorsare in e ∼ N(0, R). The focus of this discussion is on• changes to u and e and the assumptions about these terms,• the impact this has on the specification of the G structures for u and the Rstructures for e.