Causal diagrams and change variables
Article first published online: 12 SEP 2010
© 2010 Blackwell Publishing Ltd
Journal of Evaluation in Clinical Practice
Volume 18, Issue 1, pages 143–148, February 2012
How to Cite
Shahar, E. and Shahar, D. J. (2012), Causal diagrams and change variables. Journal of Evaluation in Clinical Practice, 18: 143–148. doi: 10.1111/j.1365-2753.2010.01540.x
- Issue published online: 5 JAN 2012
- Article first published online: 12 SEP 2010
- Accepted for publication: 5 August 2010
- causal diagrams;
- change score;
- change variables;
- directed acyclic graphs
Background The true change in the value of a variable between two time points is often assumed to be a cause or an effect of interest. To our knowledge, this assumption is based on intuition, rather than on any formal theoretical justification.
Methods We used causal directed acyclic graphs to explore the causal properties of a change variable, and critically examined competing structures.
Results Based on the proposed causal structure, a change variable (true change) is no more than a derived variable. It does not cause anything and is not of causal interest.
Conclusions A true change is not a variable in the physical world. Therefore, modelling the change between two time points is justified only in a few situations.