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

Dynamic prediction of response using longitudinal profi les 85<br />

= α + βT ti,j + βY y i,j + βY,Ty i,jti,j + PIi,0<br />

where αj = α + βT ti,j and βj = βY y i,j + βY,Ty i,jti,j.<br />

If the effect of the markers does not change significantly over time the interaction<br />

terms may be dropped and the model simplifies to:<br />

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

= α + βY y i,j + βT ti,j + PIi,0<br />

Here time ti,j is entered as a linear term, alternatively a smooth function of time<br />

can be used. The model can further be defined to include baseline covariates Xi,<br />

the history of markers Yi,k or include interactions over time with baseline covariates.<br />

Also changes of effect of baseline variables over time and even changes of effect of<br />

markers by baseline variables as well as covariates measured during the visits could<br />

be studied. In case of irregular time intervals between visits it is advisable to extend<br />

the model with the distance between visits to study this effect on the outcome.<br />

The model results in a set of new updated predictions depending on visit j and<br />

measurement Yi,j.<br />

The patterns of the makers as predictors<br />

Above the observed values of the markers were included directly as predictors of<br />

Ri = 1. Instead the behavior of the markers over time, for example the increase<br />

or decrease over time or some other summary measures, may be better predictors<br />

of the clinical outcome. Maruyama et al. 8 applied this idea to data on smoking<br />

cessation. We shall adapt their method in a dynamic way. Suppose the markers<br />

are observed until time T then the model is fitted in two steps. First a multivariate<br />

linear mixed model is fitted to the markers Y i =(Yi,1,...,Y i,m) observed in the<br />

interval [0, T], i.e. ti,m

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