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Chapter 2.4<br />

84<br />

a. b.<br />

c.<br />

Figure 1.The distribution of a marker at a. visit 1 and b. visit 2 by response and the bivariate distribution<br />

of the marker c. overall and d. by response. For the subject highlighted by * the marker lies as an outlier<br />

at visit 1 but not at visit 2, and stays an outlier in the bivariate distribution in c. as well as in d.<br />

logit pi,j = logit Pr(Ri =1| visit = j, y i,j,pi,0)<br />

= αj + βjy i,j + PIi,0,<br />

where PIi,0 = logitpi,0 the baseline log odds of response is used as offset. This<br />

results in j =1,...,m (the number of follow-up visits) logistic regression models<br />

with each an update of the baseline prediction. Alternatively we suggest to combine<br />

these models in a GEE manner: a pooled logistic regression analysis treating each<br />

visit as an observation and in the analysis correct for the fact that a patient is present<br />

with more visits. This idea is borrowed from Hernan and Robins, 12 who used it in<br />

their set up of the Marginal Structural Mean Model. With the inclusion of the visit<br />

time as an explanatory variable and an interaction term with the markers to estimate<br />

changes in effect of the markers over time the full GEE model, with an independent<br />

working correlation matrix, is written:<br />

d.

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