Transforming the measurements to remove a relationship between differences and magnitude

When the differences are related to magnitude, it is best to try to eliminate the relationship using a transformation. Because it has a clear interpretation, logarithmic transformation is often used to remove the effect of differences increasing with magnitude. (The difference between the logarithms of two values is equivalent to the ratio of the two values.)

Other transformations, such as square root or reciprocal, cannot be clearly interpreted and are best avoided. However, rather than use a logarithmic transformation, it is usually easier to use the ratio of the measurements or the difference expressed as a percentage of the mean.

  1. On the Fit Differences task pane, in the D drop-down list, select Ratio.
  2. Click Recalculate.
    The results are recalculated and the analysis report updates.

The difference plot shows the ratio of the measurements with the points now forming a constant-width band across the measuring interval.

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The mean ratio estimate is 1.10, which indicates that the Nadler method measures an average of 10% higher than the Hurley method. The limits of agreement indicate that the Nadler method may measure between ~6% to ~15% above the Hurley method for 95% of measurements.

(click to enlarge)