The distribution represents high density lipoprotein (HDL) cholesterol in a single subject with a true mean of 50 mg/dl and standard deviation of 9 mg/dl In the second panel we show an observed HDL cholesterol value (from this Normal distribution) of 30 mg/dl, a relatively low reading for this subject.If we were to observe another value in the same subject it would more likely be for an historical account of RTM), and it was Galton who coined the phrase ‘regression to the mean’.Conclusions RTM is a ubiquitous phenomenon in repeated data and should always be considered as a possible cause of an observed change.

We further assumed that the variation was the same for all subjects.Graphical example of true mean and variation, and of regression to the mean using a Normal distribution.The distribution represents high density lipoprotein (HDL) cholesterol in a single subject with a true mean of 50 mg/dl and standard deviation of 9 mg/dl Graphical example of true mean and variation, and of regression to the mean using a Normal distribution.Consider measuring HDL cholesterol in a random sample of subjects from a defined population.The distribution of HDL cholesterol in this population is Normal with some mean (say 60 mg/dl) and standard deviation (say 12 mg/dl).