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7 Command file: Specifying variance structures 1377.9 Constraining variance parametersParameter equality within and between variance structuresdifficultNewNewEquality of parameters in a variance model can be specified using the !=s qualifierwhere s is a string of letters and/or zeros (see Table 7.4). Positions in the stringcorrespond to the parameters of the variance model:• all parameters with the same letter in the structure are treated as the sameparameter,• 1-9 are different from a-z which are different from A-Z so that 61 equalitiescan be specified. 0 and . mean unconstrained. A colon generates a sequenceviz. a:e is the same as abcde• Putting % as the first characterin s makes the interpretation of codes absolute(so that they apply across structures).• Putting * as the first characterin s indicates that numbers are repeat counts,A-Z are equality codes and only . is unconstrained. Thus !=*.3A2. is equivalentto !=0AAA00 or !=0aaa00.Examples are presented in Table 7.5.Table 7.5: Examples of constraining variance parameters in ASRemlASReml codeaction!=ABACBA0CBAsite.gen 2 # G header linesite 0 US .3 !=0A0AA0 !GPUPUUP.1 .4 .1 .1 .3genconstrain all parameters correspondingto A to be equal, similarly for B and C.The 7th parameter would be left unconstrained.This sequence applied to anunstructured 4 × 4 matrix would makeit banded, that isAB AC B A0 C B Athis example defines a structure for thegenotype by site interaction effects in aMET in which the genotypes are independentrandom effects within sites butare correlated across sites with equal covariance.