title: Analysing cognitive test data: Distributions and non-parametric random effects.
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.
authors.affiliation: MRC Unit for Lifelong Health and Aging, London, UK g.muniz@nshd.mrc.ac.uk.
authors.forename: Graciela
authors.initials: G
authors.lastname: Muniz-Terrera
authors.affiliation: Department of Statistical Science, University College London, UK.
authors.forename: Ardo van den
authors.initials: Av
authors.lastname: Hout
authors.affiliation: STORM Research Centre, London Metropolitan University, UK.
authors.forename: R A
authors.initials: RA
authors.lastname: Rigby
authors.affiliation: STORM Research Centre, London Metropolitan University, UK.
authors.forename: D M
authors.initials: DM
authors.lastname: Stasinopoulos
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D003071:Cognition; D060825:Cognitive Dysfunction / Q000175:diagnosis*; D005260:Female; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D018709:Statistics, Nonparametric; D011795:Surveys and Questionnaires
keywords: Cognitive test; beta binomial; random effects models