Bland–Altman plot: A brief overview

In healthcare research, it is common to compare two methods of measurement to determine the overall degree of agreement. The Bland–Altman (BA) plot is an alternative to traditional correlational analyses. The BA plot portrays the agreement graphically by creating statistical limits of agreement using the mean and standard deviation of the differences between two measurements. The difference (Test #1 − Test #2) is constructed on the vertical axis while the mean ([Test #1 + Test #2]/2) is depicted on the horizontal axis. Within this plot, one can detect bias between the mean differences, as well as estimate an agreement interval. If the data points are normally distributed, 95% of differences will lie between the limits, but smaller sample sizes may be unreliable for estimating larger population parameters. Although nonparametric methods can estimate limits of agreement with nonnormally distributed data, they may be less reliable than logarithmically transforming the data before creating the plot. The following core competencies are addressed in this article: Practice-based learning and improvement, Medical knowledge.