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Biostatistics Advance Access originally published online on April 14, 2005
Biostatistics 2005 6(3):479-485; doi:10.1093/biostatistics/kxi023
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© The Author 2005. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org.

Exploratory analysis of longitudinal trials with staggered intervention times

Inês Sousa*, Amanda G. Chetwynd and Peter J. Diggle

Department of Mathematics and Statistics, Lancaster University, LA1 4YF, Lancaster, UK i.pereirasilvacunhasousa{at}lancaster.ac.uk

* To whom correspondence should be addressed.

Longitudinal trials involving surgical interventions commonly have subject-specific intervention times, due to constraints on the availability of surgeons and operating theatres. Moreover, the intervention often effects a discontinuous change in the mean response. We propose a nonparametric estimator for the mean response profile of longitudinal data with staggered intervention times and a discontinuity at the times of intervention, as an exploratory tool to assist the formulation of a suitable parametric model. We use an adaptation of the standard generalized additive model algorithm for estimation, with smoothing constants chosen by a cross-validation criterion. We illustrate the method using longitudinal data from a trial to assess the effect of lung resection surgery in the treatment of emphysema patients.

Keywords: Back-fitting algorithm; Cross-validation; Exploratory analysis; Longitudinal trials; Lung resection surgery; Nonparametric estimator


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