Bland-Altman plot

The Bland-Altman plot shows how much two measurements of the same cases differ. The horizontal axis holds the mean of both values, the vertical axis their difference. The bias is the mean difference (a constant offset between the methods). The limits of agreement, bias ± 1.96 SD, enclose about 95% of the differences. Whether the range is small enough is for the field to decide, not statistics.

Example: Two scales measure 15 samples: bias = −0.33 (95% CI −0.98 to 0.32), limits of agreement −2.64 to 1.97. There is no clear evidence of a constant offset between the scales (the CI of the bias includes 0); for a single sample, however, they can differ by more than two units.

Caution: The limits are imprecise with few cases (Bland and Altman recommend at least 100). If the differences depend on the mean, the limits do not hold equally well everywhere. The ICC relates the disagreement to the differences between cases; the plot shows it in the unit of measurement.