Decoding the Bland–Altman plot: Basic review

Abstract

The Bland–Altman plot is a method for comparing two measurements of the same variable. The concept is that X-axis is the mean of your two measurements, and the Y-axis is the difference between the two measurements. The chart can then highlight anomalies, for example, if one method always gives too high a result, then all points are above or below the zero line. It can also reveal that one method overestimates high values and underestimates low values. If the points on the Bland–Altman plot are scattered all over the place, above and below zero, then it suggests that there is no consistent bias of one approach versus the other. It is, therefore, a good first step for two measurement techniques of a variable.