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6 Command file: Specifying the terms in the mixed model 91Conditional factorsA conditional factor is a factor that is present only when another factor has aparticular level.New• individual components can be specified using the at(f,n) function (see Table6.2), for example, at(site,1).row will fit row as a factor only for site 1,• a complete set of conditional terms are specified by omitting the level specificationin the at(f) function provided the correct number of levels of f isspecified in the field definitions. Otherwise, a list of levels may be specified.– at(f).b creates a series of model terms representing b nested within a forany model term b. A model term is created for each level of a; each hasthe size of b. For example, if site and geno are factors with 3 and 10 levelsrespectively, then for at(site).geno ASReml constructs 3 model termsat(site,1).geno at(site,2).geno at(site,3).geno, each with 10 levels,– this is similar to forming an interaction except that a separate model term iscreated for each level of the first factor; this is useful for random terms wheneach component can have a different variance. The same effect is achievedby using an interaction (e.g. site.geno) and associating a DIAG variancestructure with the first component (see Section 7.5).6.6 Alphabetic list of model functionsTable 6.2 presents detailed descriptions of the model functions discussed above.Note that some three letter function names may be abbreviated to the first letter.Table 6.2: Alphabetic list of model functions and descriptionsmodel functionactionand(t,r)a(t,r)overlays (adds) r times the design matrix for model term t to the existingdesign matrix. Specifically, if the model up to this point has p effects andt has a effects, the a columns of the design matrix for t are multiplied bythe scalar r (default value 1.0) and added to the last a of the p columnsalready defined. The overlaid term must agree in size with the term itoverlays. This can be used to force a correlation of 1 between two termsas in a diallel analysismale and(female)assuming the ith male is the same individual as the ith female. Note thatif the overlaid term is complex, it must be predefined; e.g.Tr.male -Tr.female and(Tr.female).