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Biostatistics Advance Access first published online on April 14, 2005
This version published online on April 22, 2005
Biostatistics, 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.
Received October 25, 2004
Revised February 25, 2005
Accepted February 25, 2005

Article

Exploratory analysis of longitudinal trials with staggered intervention times

Inês Sousa 1*, Amanda G. Chetwynd 1, and Peter J. Diggle 1

1 Department of Mathematics and Statistics, Lancaster University, LA1 4YF, Lancaster, UK

* To whom correspondence should be addressed.
Inês Sousa, E-mail: i.pereirasilvacunhasousa{at}lancaster.ac.uk


  Abstract

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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