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Place Diff in the "Dependent" box and MMEAN in the "Independent(s)" box and click OK.
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Here are the syntax commands for the transformation:: compute diff = ln(A) - ln(B). The ln() function returns the natural logarithm of the variable or number in the parentheses.
The remaining commands in the example would remain unchanged.
If the Bland-Altman plot indicates that the variance of DIFF varies across the range of MMEAN, e.g., if the vertical spread of scatter points is much narrower at low values of MMEAN than at high values of MMEAN, Bland and Altman suggest that the researcher calculate the natural logarithms of the two measures (A and B in this example), recalculate the difference and mean measures with new variables and redraw the Bland-Altman plot with the new difference and mean measures.
The new DIFF and MMEAN could be calculated in the Transform-Compute dialogs as before with slightly more complex numeric expressions.
In the Bland-Altman plot, this bias will be reflected in the scatter points, with a trend to higher or lower values of DIFF across the range of values of MMEAN.