Bland-Altman Plot (Bias and Limit of Agreement)

The Bland-Altman Plot is a statistical visualization tool that is widely used for comparing two sets of measurements. It is named after J. Martin Bland and Douglas G. Altman, who developed it in 1986. The plot is also known as the Tukey Mean-Difference Plot, the Difference Plot, or the Limits of Agreement Plot. The plot is an essential tool for anyone involved in biomedical research, clinical trials, or medical practice, as it allows for a quick assessment of the agreement between two methods of measuring a variable, the bias, and the limits of agreement of the measurements.

The Bland-Altman plot is a scatter plot that shows the differences between two measurements (y-axis) against their average (x-axis). The plot is accompanied by two lines, one representing the bias (the mean difference between the two methods of measurement), and the other representing the limit of agreement (the range of values within which 95% of the differences between the two methods fall). The plot can also show outliers, which are cases where the difference between the two methods is much larger than expected.

The plot allows researchers and practitioners to identify any systematic bias between the two methods of measurement. A systematic bias occurs when one method consistently provides higher or lower measurements than the other method. The bias line in the plot helps to identify the direction and magnitude of the bias. If the line is close to zero, there is no significant bias between the two methods. If the line is above or below zero, there is a systematic bias, and the offset from zero represents the size of the bias.

The plot also allows for the assessment of the agreement between the two methods of measurement. The limit of agreement line shows the range of values within which 95% of the differences between the two methods are expected to fall. If the two methods have good agreement, the line will be narrow, indicating that the differences between the methods are consistent. If the line is wide, it indicates poor agreement between the two methods, and the differences between the methods are not consistent.

In conclusion, the Bland-Altman plot is a valuable tool for assessing the agreement between two methods of measuring a variable, identifying any systematic bias, and determining the limits of agreement. It is commonly used in biomedical research, clinical trials, and medical practice, and is a useful addition to any statistical analysis. By understanding the basics of the Bland-Altman plot, researchers and practitioners can gain valuable insights into the precision and accuracy of their measurement methods.