When assessing the agreement between two methods, it is useful to the plot the
difference between methods against the mean of the methods (a difference plot).
difference plot is, effectively, a scatter plot rotated 45 degrees clockwise. However, a
difference plot is more informative than a scatter plot since the data points are not
tightly clustered around the diagonal. A difference plot also clearly shows any
relationship between the differences and the magnitude of measurement.
illustrate this concept, we will use an example from Bland & Altman’s 1986 paper.
This example compares two methods of measuring peak expiratory flow rate (PEFR). On each
of 17 subjects, two measurements were taken with a Wright meter, and two with a mini