Email updates

Keep up to date with the latest news and content from BMC Medical Research Methodology and BioMed Central.

Open Access Research article

Visualising and modelling changes in categorical variables in longitudinal studies

Mark Jones12*, Richard Hockey1, Gita D Mishra1 and Annette Dobson1

Author Affiliations

1 Centre for Longitudinal and Life Course Research, School of Population Health, University of Queensland, Brisbane, Australia

2 Public Health Building, Herston Road Herston, Brisbane, Qld, 4006, Australia

For all author emails, please log on.

BMC Medical Research Methodology 2014, 14:32  doi:10.1186/1471-2288-14-32

Published: 27 February 2014

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.

Keywords:
Categorical variables; Graphical methods; Longitudinal studies; Marginal distribution; Nominal regression; Transition probabilities