Visualising and modelling changes in categorical variables in longitudinal studies

Jones, Mark, Hockey, Richard, Mishra, Gita D. and Dobson, Annette (2014) Visualising and modelling changes in categorical variables in longitudinal studies. BMC Medical Research Methodology, 14 1: 32.1-32.8. doi:10.1186/1471-2288-14-32

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Author Jones, Mark
Hockey, Richard
Mishra, Gita D.
Dobson, Annette
Title Visualising and modelling changes in categorical variables in longitudinal studies
Journal name BMC Medical Research Methodology   Check publisher's open access policy
ISSN 1471-2288
Publication date 2014-02-27
Sub-type Article (original research)
DOI 10.1186/1471-2288-14-32
Open Access Status DOI
Volume 14
Issue 1
Start page 32.1
End page 32.8
Total pages 8
Place of publication London, United Kingdom
Publisher BioMed Central
Language eng
Subject 2713 Epidemiology
2718 Health Informatics
Formatted abstract
Background: Graphical techniques can provide visually compelling insights into complex data patterns. In this paper we present a type of lasagne plot showing changes in categorical variables for participants measured at regular intervals over time and propose statistical models to estimate distributions of marginal and transitional probabilities.

Methods: The plot uses stacked bars to show the distribution of categorical variables at each time interval, with different colours to depict different categories and changes in colours showing trajectories of participants over time. The models are based on nominal logistic regression which is appropriate for both ordinal and nominal categorical variables. To illustrate the plots and models we analyse data on smoking status, body mass index (BMI) and physical activity level from a longitudinal study on women's health. To estimate marginal distributions we fit survey wave as an explanatory variable whereas for transitional distributions we fit status of participants (e.g. smoking status) at previous surveys.

Results: For the illustrative data the marginal models showed BMI increasing, physical activity decreasing and smoking decreasing linearly over time at the population level. The plots and transition models showed smoking status to be highly predictable for individuals whereas BMI was only moderately predictable and physical activity was virtually unpredictable. Most of the predictive power was obtained from participant status at the previous survey. Predicted probabilities from the models mostly agreed with observed probabilities indicating adequate goodness-of-fit.

Conclusions: The proposed form of lasagne plot provides a simple visual aid to show transitions in categorical variables over time in longitudinal studies. The suggested models complement the plot and allow formal testing and estimation of marginal and transitional distributions. These simple tools can provide valuable insights into categorical data on individuals measured at regular intervals over time. 
Keyword Categorical variables
Graphical methods
Longitudinal studies
Marginal distribution
Nominal regression
Transition probabilities
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: Official 2015 Collection
School of Public Health Publications
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