This plot shows diastolic blood pressure from a hypothetical "before and after" study:

Subject A has DBP = 100 on occasion 1, DBP = 90 on occasion 2

Subject B has DBP = 110 on occasion 1, DBP = 110 on occasion 2

Subject C has DBP = 90 on occasion 1, DBP = 100 on occasion 2

Subject D has DBP = 80 on occasion 1, DBP = 80 on occasion 2

A horizontal line is drawn through the mean DBP, 95 mm Hg. The Occasion 1 and Occasion 2 points are connected for each subject.

The mean is the same on both occasions. If we select only the subjects above the mean line, A and B,their mean will be less on the second occasion than on the first.

Back to Regression towards the mean.

This page maintained by Martin Bland.

Last updated: 19 January, 2004.