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A difference plot shows the differences between two observations on the same sampling unit.
The difference plot shows the difference between two observations on the vertical axis against the average of the two observations on the horizontal axis. A gray identity line represents equality; no difference.
If the second observation is always greater than the first the points lie above the line of equality, or vice-versa. If differences are not related to the magnitude the points will form a horizontal band. If the points form an increasing, decreasing, or non-constant width band, then the variance is not constant.
It is common to combine the difference plot with a histogram and a normality plot of the differences to check if the differences are normally distributed, which is an assumption of some statistical tests and estimators.