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Analysing cognitive test data: Distributions and non-parametric random effects

Muniz-Terrera, G; Hout, AVD; Rigby, R; Stasinopoulos, D; (2016) Analysing cognitive test data: Distributions and non-parametric random effects. Statistical Methods in Medical Research , 25 (2) pp. 741-753. 10.1177/0962280212465500. Green open access

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Abstract

An important assumption in many linear mixed models is that the conditional distribution of the response variable is normal. This assumption is violated when the models are fitted to an outcome variable that counts the number of correctly answered questions in a questionnaire. Examples include investigations of cognitive decline where models are fitted to Mini Mental State Examination scores, the most widely used test to measure global cognition. Mini Mental State Examination scores take integer values in the 0–30 range, and its distribution has strong ceiling and floor effects. This article explores alternative distributions for the outcome variable in mixed models fitted to mini mental state examination scores from a longitudinal study of ageing. Model fit improved when a beta-binomial distribution was chosen as the distribution for the response variable.

Type: Article
Title: Analysing cognitive test data: Distributions and non-parametric random effects
Open access status: An open access version is available from UCL Discovery
DOI: 10.1177/0962280212465500
Publisher version: http://dx.doi.org/10.1177/0962280212465500
Language: English
Additional information: Copyright © The Author(s) 2012. The published version of record is available from SAGE journals at http://smm.sagepub.com/content/25/2/741
Keywords: Cognitive test; beta binomial; random effects models
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Pop Health Sciences > Institute of Cardiovascular Science
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Statistical Science
URI: https://discovery.ucl.ac.uk/id/eprint/1450523
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