General Practice

Statistics Notes: Some examples of regression towards the mean

BMJ 1994; 309 doi: (Published 24 September 1994) Cite this as: BMJ 1994;309:780
  1. J M Bland,
  2. D G Altman
  1. Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE Medical Statistics Laboratory, Imperial Cancer Research Fund, London WC2A 3PX.

    We have previously shown that regression towards the mean occurs whenever we select an extreme group based on one variable and then measure another variable for that group (4 June, p 1499).1 The second group mean will be closer to the mean for all subjects than is the first, and the weaker the correlation between the two variables the bigger the effect will be. Regression towards the mean happens in many types of study. The study of heredity1 is just one. Once one becomes aware of the regression effect it seems to be everywhere. The following are just a few examples.

    Treatment to reduce high levels of a measurement - In clinical practice there are many measurements, such as weight, serum cholesterol concentration, or blood pressure, for which particularly high or low values are signs of underlying disease or risk factors for disease. People with extreme values of the measurement, such as high blood pressure, may be treated to bring their values closer to the mean. If they are measured again we will observe that the mean of the extreme group is now closer to the mean of the whole population - that is, it …

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