View PDF Version - RePub - Erasmus Universiteit Rotterdam
View PDF Version - RePub - Erasmus Universiteit Rotterdam
View PDF Version - RePub - Erasmus Universiteit Rotterdam
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
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