Journal
STATISTICAL METHODS IN MEDICAL RESEARCH
Volume 25, Issue 2, Pages 741-753Publisher
SAGE PUBLICATIONS LTD
DOI: 10.1177/0962280212465500
Keywords
Cognitive test; beta binomial; random effects models
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Funding
- MRC [MC_U123092720] Funding Source: UKRI
- Medical Research Council [MC_U123092720] Funding Source: Medline
- Medical Research Council [MC_U123092720] Funding Source: researchfish
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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.
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