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7 Command file: Specifying variance structures 1157.4 Variance structuresThe previous sections have introduced variance modelling in ASReml using theNIN data for demonstration. In this and the remaining sections the syntax isdescribed formally. However, where appropriate we continue to reference theexample.General syntaxVariance model specification in ASReml has the following general form[variance header line[R structure definition lines ][G structure header and definition lines ][variance parameter constraints ]]• variance header line specifies the number of R and G structures,• R structure definition lines define the R structures (variance models for error)as specified in the variance header line,• G structure header and definition lines define the G structures (variance modelsfor the additional random terms in the model) as specified in the varianceheader line; these lines are always placed after any R structure definition lines,• variance parameter constraints are included if parameter constraints are to beimposed, see the !VCC c qualifier in Table 5.5 and Section 7.9 on constraintsbetween and within variance structures.A schematic outline of the variance model specification lines (variance headerline, and R and G structure definition lines) is presented in Table 7.2 using thevariance model of 4 for demonstration.