A type of plot originally suggested by Tukey, and independently introduced by Altman and Bland in 1983 in a medical context. Measurements of a characteristic (e.g. blood pressure) are made independently by two observers (or two instruments) on a series of n individuals; denoting these measurements by (x₁₁, x₂₁), … (x_1n, x_2n), the plot is a scatter diagram with (x_1i−x_2i) giving the vertical coordinate and (x_1i+x_2i)/2 giving the horizontal coordinate. The plot is also known as Tukey’s mean-difference plot or MD-plot.