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7 Command file: Specifying variance structures 117Variance header lineThe variance header line is of the form[s [c [g]]]• s and c relate to the R structures, g is thenumber of G structures,• the variance header line may be omittedif the default IID R structure is required,no G structures are being explicitly definedand there are no parameter constraints (see!VCC and examples 1 and 2a),• s is used to code the number of independentsections in the error termNIN Alliance Trial 1989variety !Aid.row 22column 11nin89aug.asd !skip 1yield ∼ mu variety !r repl,!f mv1 2 122 row AR1 0.311 column AR1 0.3repl 1repl 0 IDV 0.1– if s = 0, the default IID R structure is assumed and no R structure definitionlines are required (as in examples 2b and 5),– if s > 0, s R structure definitions are required, one for each of the s sections(as in examples 3a, 3b, 3c and 4),– for the analysis of multi-section data s can be replaced by the name of afactor with the appropriate number of levels, one for each section,• c is the number of component variance models involved in the variance structurefor the error term for each section; for example, 3a, 3b and 3c havecolumn.row as the error term and the variance structure for column.row involves2 variance models, the first for column and the second for row,– c has a default value of 2 when s is not specified as zero,• g is the number of variance structures (G structures) that will be explicitlyspecified for the random terms in the model.See Table 7.3See Section 7.7R and G structures are now discussed with reference to s, c and g. As alreadynoted, each variance structure may involve several variance models which relateto the individual terms involved in the random effect or error. For example, atwo factor interaction may have a variance model for each of the two factorsinvolved in the interaction. Variance models are listed in Table 7.3. As indicatedin the discussion of 2b, care must be taken with respect to scale parameters whencombining variance models (see also Section 7.7).