Regression to the mean: what it is and how to deal with it

Barnett, Adrian G
van der Pols, Jolieke C
Dobson, Annette J

¹School of Population Health, The University of Queensland, Herston, Brisbane QLD 4006, Australia

Accepted 1 July 2004

Correspondence: Adrian Barnett, School of Population Health, The University of Queensland, Brisbane QLD 4006, Australia. E-mail: [email protected]

International Journal of Epidemiology 34(1):p 215-220, February 2005.

Background

Regression to the mean (RTM) is a statistical phenomenon that can make natural variation in repeated data look like real change. It happens when unusually large or small measurements tend to be followed by measurements that are closer to the mean.

Methods

We give some examples of the phenomenon, and discuss methods to overcome it at the design and analysis stages of a study.

Results

The effect of RTM in a sample becomes more noticeable with increasing measurement error and when follow-up measurements are only examined on a sub-sample selected using a baseline value.

Conclusions

RTM is a ubiquitous phenomenon in repeated data and should always be considered as a possible cause of an observed change. Its effect can be alleviated through better study design and use of suitable statistical methods.