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7 Command file: Specifying variance structures 139• the P i terms refer to positions in the full variance parameter vector. This maychange if the model is changed and is often difficult to determine as a number.If it refers to a parameter which is a single traditional variance componentassociated with a random term, the name of the random term may be giveninstead of the parameter number. The full parameter vector includes a termfor each factor in the model and then a term for each parameter defined in theR and G structures. A list of P i numbers and their initial values is returnedin the .res file to help you to check the numbers. Alternatively, examine the.asr file from an initial run with !VCC included but no arguments supplied.The job will terminate but ASReml will provide the P i values associated witheach variance component. Otherwise the numbers are given in the .res file.The following are examples:ASReml codeaction5 7 * .1 parameter 7 is a tenth of parameter 55 -7 parameter 7 is the negative of parameter 532 34 35 37 38 39 for a (4 × 4) US matrix given by parameters 31 . . . 40,the covariances are forced to be equal.units -uni(check)parameter associated with model term uni(check)has the same magnitude but opposite sign to the parameterassociated with model term units7.10 Model building using the !CONTINUE qualifierdifficultIn complex models, the Average Information algorithm can have difficulty maximisingthe REML log-likelihood when starting values are not reasonably closeto the REML solution. ASReml has several internal strategies to cope with thisproblem but these are not always successful.When the user needs to provide better starting values, one method is to fit asimpler variance model. For example, it can be difficult to guess reasonablestarting values for an unstructured variance matrix. A first step might be toassume independence and just estimate the variances. If all the variances are notpositive, there is little point proceeding to try and estimate the covariances.