Statistical methods for assessing agreement between two methods of clinical measurement. The reference is:īland, J.M., & Altman, D.G. 'Small' would be an amount that would be clinically insignificant for the factor being measured. (Bland and Altman also discuss the option of using confidence interval bounds, based on the standard error of the mean, for the upper and lower reference lines.) If the two methods are comparable, then differences should be small, with the mean of the differences close to 0, and show no systematic variation with the mean of the two measurements.
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Three horizontal reference lines are superimposed on the scatterplot - one line at the average difference between the measurements, along with lines to mark the upper and lower control limits of plus and minus 1.96*sigma, respectively, where sigma is the standard deviation of the measurement differences. The Bland-Altman chart is a scatterplot with the difference of the two measurements for each sample on the vertical axis and the average of the two measurements on the horizontal axis.
Data for the plot would be collected by gathering a number of blood samples, splitting each in two, and measuring the factor using both methods. Say one wanted to compare two techniques of measuring some blood factor. Suppose there are two techniques for measuring some continuously-scaled variable, each having some error, and we want a graphical means to assess whether or not they are comparable. The Bland-Altman plot (Bland & Altman, 1986) is most likely to be seen in the medical statistics literature.
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