Illustration of invariance to time origin

We pointed out in Section 2.1.4 that the fixed effects and V matrix estimates were invariant to the origin used for time. We illustrate this property by fitting Model 1 with time centred about its mean of 10 months (i.e. using time = time — 10). Results from this model are shown in Table 6.7.

Thus, the variance parameters connected with the patient random coefficients (intercepts) and the fixed intercept effect have altered to adjust for the change in time origin. However, the fixed effects and the other variance parameters are unaltered. The terms in V = var(y) will also be found to be identical between the models when calculated for specific values of time.

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