Akacha, Mouna and Hutton, Jane (2009) Analysing the rate of change in a longitudinal study with missing data, taking into account the number of contact attempts. Working Paper. Coventry: University of Warwick. Centre for Research in Statistical Methodology. Working papers, Vol.2009 (No.42).

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Abstract

In longitudinal and multivariate settings incomplete data, due to missed visits, dropouts or non-return of
questionnaires are quite common.
A longitudinal trial in which potentially informative missingness occurs is the Collaborative Ankle Support
Trial (CAST). The aim of this study is to estimate the clinical effectiveness of four different methods of
mechanical support after severe ankle sprain. The clinical status of multiple subjects was measured at four
points in time via a questionnaire and, based on this, a continuous and bounded outcome score was calculated.
Motivated by this study, a model is proposed for continuous longitudinal data with non-ignorable or
informative missingness, taking into account the number of attempts made to contact initial non-responders.
The model combines a non-linear mixed model for the underlying response model with a logistic regression
model for the reminder process.
The outcome model enables us to analyze the rate of improvement including the dependence on explanatory
variables. The non-linear mixed model is derived under the assumption that the rate of improvement in a given
time interval is proportional to the current score and the still achievable score. Based on this assumption a
differential equation is solved in order to obtain the model of interest.
The response model relates the probability of response at each contact attempt and point in time to
covariates and to observed and missing outcomes.
Using this model the impact of missingness on the rate of improvement is evaluated for different missingness
processes.

Item Type:	Working or Discussion Paper (Working Paper)
Subjects:	Q Science > QA Mathematics
Divisions:	Faculty of Science, Engineering and Medicine > Science > Statistics
Library of Congress Subject Headings (LCSH):	Missing observations (Statistics), Medical statistics, Clinical trials -- Mathematical models
Series Name:	Working papers
Publisher:	University of Warwick. Centre for Research in Statistical Methodology
Place of Publication:	Coventry
Official Date:	2009
Dates:	Date Event 2009 Published
Volume:	Vol.2009
Number:	No.42
Number of Pages:	20
Institution:	University of Warwick
Status:	Not Peer Reviewed
Access rights to Published version:	Open Access (Creative Commons)
Date of first compliant deposit:	1 August 2016
Date of first compliant Open Access:	1 August 2016

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