Validating the bland altman method of agreement
Choose "Edit Content Y-axis Reference Line to add each line. Alternatively, you could use the GUIDE command in the GPL command sequence to directly request the three reference lines and avoid the need for the chart editor. GGRAPH /GRAPHDATASET NAME="graphdataset" VARIABLES=mmean diff MISSING=LISTWISE REPORTMISSING=NO /GRAPHSPEC SOURCE=INLINE.
A simple test for a linear trend of this sort would be to run the Linear Regression procedure (Analyze-Linear).
It is identical to a Tukey mean-difference plot Bland and Altman make the point that any two methods that are designed to measure the same parameter (or property) should have good correlation when a set of samples are chosen such that the property to be determined varies considerably.
A high correlation for any two methods designed to measure the same property could thus in itself just be a sign that one has chosen a widespread sample.
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.Place Diff in the "Dependent" box and MMEAN in the "Independent(s)" box and click OK.