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The difference plot may sometimes suggest a relationship between the differences and
magnitude of measurement. Often, the band of points on the difference plot will start narrow
and then widen to the right of the plot as magnitude increases. This indicates that the
variability of the differences increases with magnitude of measurement. Sometimes, the band
of points may not be horizontal, which indicates a relationship between average difference
and the magnitude.
To illustrate these concepts, we will use an example from Bland &
Altman’s 1999 paper. This example measures plasma volume expressed as a percentage of
normal using two alternative sets of normal values from Nadler and Hurley.
The difference plot shows an obvious systematic difference between the methods as all
the differences are above the line of equality (zero). There is also a clear
relationship between the difference and the mean, with the difference increasing as
plasma volume rises. The limits of agreement appear rather wide at low values and
too narrow at higher values.