# Plotting the relationship between methods

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).

A 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.

To 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 Wright meter.

The points on the difference plot roughly form a constant width horizontal band across the measuring interval, and there is no obvious relationship between the difference and the mean or the variability of the measurements and the mean.

- Tutorials
- Distribution tutorial
- Correlation / PCA tutorial
- Compare groups means tutorial
- Association in 2-way contingency tables tutorial
- Simple linear regression tutorial
- Bland-Altman method comparison tutorial
- Plotting the relationship between methods
- Estimating the average bias and the limits of agreement
- Understanding the importance of repeatability
- Using repeated measurements
- Dealing with a relationship between difference and magnitude of measurement
- Transforming the measurements to remove a relationship between differences and magnitude
- Estimating regression based limits of agreement when transformation is not enough
- Estimating nonparametric limits of agreement in non-normally distributed data
- Pareto charts tutorial
- Process control charts tutorial
- Process capability tutorial

Published 8-Jan-2017

Version 4.90