Within-subject covariance structure

At this stage, different within-subject covariance structures, along with between-subject covariances, can... 

Table 6.2 Structures of common within-subject covariance (6.27) structures assuming three measurements per subject.

An alternative method to treating time as a categorical variable is to treat time as a continuous variable and to model time using a low-order polynomial, thereby reducing the number of estimable parameters in the model. In this case, rather than modeling the within-subject covariance the between-subject covariance is manipulated. Subjects are treated as random effects, as are the model parameters associated with time. The within-subject covariance matrix was treated as a simple covariance structure. In this example, time was modeled as a quadratic polynomial. Also, included in the model were the interactions associated with the quadratic term for time. 

F rom an analysis of variance with a general linear model including the effect of diet (D) and batch (B), lactation stage (S) and their interactions as fixed effects. Repeated measurements of milk chemical composition were analysed using an unstructured covariance structure wilh sows within bateh as a subject. Statistical significance P<0.05, ... 

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