Bland–Altman plot

A Bland–Altman plot (Difference plot) in analytical chemistry or biomedicine is a method of data plotting used in analyzing the agreement between two different assays. It is identical to a Tukey mean-difference plot, the name by which it is known in other fields, but was popularised in medical statistics by J. Martin Bland and Douglas G. Altman.Bland and Altman make the point that any two methods that are designed to measure the same parameter (or property) should have good correlation when a set of samples are chosen such that the property to be determined varies considerably. A high correlation for any two methods designed to measure the same property could thus in itself just be a sign that one has chosen a widespread sample. A high correlation does not necessarily imply that there is good agreement between the two methods.Consider a set of n {displaystyle n}   samples (for example, objects of unknown volume). Both assays (for example, different methods of volume measurement) are performed on each sample, resulting in 2 n {displaystyle 2n}   data points. Each of the n {displaystyle n}   samples is then represented on the graph by assigning the mean of the two measurements as the x {displaystyle x}  -value, and the difference between the two values as the y {displaystyle y}  -value.One primary application of the Bland–Altman plot is to compare two clinical measurements each of which produced some error in their measures. It can also be used to compare a new measurement technique or method with a gold standard, as even a gold standard does not—and should not—imply it to be without error. See Analyse-it, MedCalc, NCSS, GraphPad Prism, R, or StatsDirect for software providing Bland–Altman plots.A similar method was proposed in 1981 by Eksborg. This method was based on Deming regression—a method introduced by Adcock in 1878.