Plot of predicted virus antibody level

The results become more meaningful when the predicted antibody level is plotted against time. This is done for Models 2 and 3 with the outlying observation removed (Figure 6.7). The quadratic model (Model 2) curve is also plotted to

Figure 6.6 Plot of random coefficients: (a) patient coefficients vs. patient-time coefficients; (b) patient coefficients vs. patient-time2 coefficients; (c) patient-time coefficients vs. patient-time2 coefficients. A = 1 obs., B = 2 obs., etc.

PT_RESID

A AAA AAB

0.000 PT2_RESI

0.005

0.010

Figure 6.6 (continued).

MONTH

Figure 6.7 Predicted virus antibody level vs. time (Models 2 and 3). Curve:_, cubic;

MONTH

Figure 6.7 Predicted virus antibody level vs. time (Models 2 and 3). Curve:_, cubic;

Table 6.7 Results from Model 1 with time centred about its mean of 10 months.

Model

Fixed effects

G matrix and residual

1 - with altered time origin Intercept 3.34(0.2 7)

0.44 0.013

0.56

1.13 0.056

0.56

0.0042

0.0042

make the conclusions more robust since the cubic coefficient is of dubious significance.

The curves for the two models differ markedly for higher values of time. However, only a small proportion of observations were made after 24 months (5%) and the cubic coefficient will be based largely on these. The models are therefore only really plausible up to 24 months, for which they are quite similar. The virus antibody levels decrease most rapidly initially, then flatten. The divergent curves illustrate how over-interpretation might occur when part of them is only based on a small amount of data.

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