Abstract
We present a simple semiparametric model for fitting subject-specific
curves for longitudinal data. Individual curves are modelled as penalized
splines with random coefficients. This model has a mixed model representation,
and it is easily implemented in standard statistical software. We
conduct an analysis of the long-term effect of radiation therapy
on the height of children suffering from acute lymphoblastic leukaemia
using penalized splines in the framework of semiparametric mixed
effects models. The analysis revealed significant differences between
therapies and showed that the growth rate of girls in the study cannot
be fully explained by the group-average curve and that individual
curves are necessary to reflect the individual response to treatment.
We also show how to implement these models in S-PLUS and R in the
appendix. Copyright © 2004 John Wiley & Sons, Ltd.
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