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6 Command file: Specifying the terms in the mixed model 96Alphabetic list of model functions and descriptionsmodel functionactionuni(f,k[,n])xfa(f,k)creates a factor with a level for every record subject to the factor levelof f equalling k, i.e. a new level is created for the factor whenever anew record is encountered whose integer truncated data value from datafield f is k. Thus uni(site,2) would be used to create an independenterror term for site 2 in a multi-environment trial and is equivalent toat(site,2).units. The default size of this model term is the number ofdata records. The user may specify a lower number as the third argument.There is little computational penalty from the default but the .sln filemay be substantially larger than needed.Factor analytic models are discussed in Chapter 7. There are three forms,FAk, FACVk and XFAk where k is the number of factors. The XFAk form is asparse formulation that requires an extra k levels to be inserted into themixed model equations for the k factors. This is achieved by the xfa(f,k)model function which defines a design matrix based on the design matrixfor f augmented with k columns of zeros for the k factors.6.7 WeightscautionWeighted analyses are achieved by using !WT weight as a qualifier to the responsevariable. An example of this is y !WT wt ∼ mu A X where y is the name of theresponse variable and wt is the name of a variate in the data containing weights.If these are relative weights (to be scaled by the units variance) then this is allthat is required. If they are absolute weights, that is, the reciprocal of knownvariances, use the !S2==1 qualifier described in Table 7.4 to fix the unit variance.When a structure is present in the residuals the weights are applied as a matrixproduct. If Σ is the structure and W is the diagonal matrix constructed from thesquare root of the values of the variate weight, then R −1 = W Σ −1 W . Negativeweights are treated as zeros.6.8 Generalized Linear ModelsASReml includes facilities for fitting the family of Generalized Linear Models(GLMs, McCullagh and Nelder, 1994). GLMs are specified by qualifiers after thename of the dependent variable but before the ∼ character. Table 6.3 lists the link