[R] help with Tukey Mean-Difference Plot
f.mattes at ucl.ac.uk
Fri Aug 15 23:15:36 CEST 2003
Dear R users,
I would appreciate for some advise how to generate a Tukey
Mean-Difference Plot with the tmd function part of the lattice
library. I have two test results (log transformed) which showing a
correlation on a scatterplot. However the correlation line is
parallel displaced depending on a clinical condition. I thought to a
Tukey Mean Difference Plot would show me the difference betweenthe
I don't have the literature how the plot gets constructed, but my understanding
is that the first from each distribution a qq plot is constructed and
than the coardiantes of the qq-plot are transformed according the
formula x=x+y/2 and y=x+y.
I tried this with
tmd(xyplot(a ~ b| c, data=data)) with
a and b holding my log10 transformed numerical values, and c the condition.
However I think I don't get a Tukey Mean-Difference plot. It looks to
me that only
x=x+y/2 and y=x+y. transformation is performed (a=a+b; b=b+a/2).
and the result is displayed as lattice graphic.
It would be great if anyone could help me to find the error in my plot.
Frank Mattes e-mail: f.mattes at ucl.ac.uk
Department of Virology fax 0044(0)207 8302854
Royal Free Hospital and tel 0044(0)207 8302997
University College Medical School
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