# [R] help with Tukey Mean-Difference Plot

Thomas W Blackwell tblackw at umich.edu
Fri Aug 15 23:43:14 CEST 2003

```Frank  -

help("tmd")  says that in R, the Tudkey Mean Difference plot is
constructed by:  x=(x+y)/2,  y=y-x.  That's different from what
you quote below, but it agrees with my memory of it.  Not sure

-  tom blackwell  -  u michigan medical school  -  ann arbor  -

On Fri, 15 Aug 2003, Frank Mattes wrote:

> 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
> two values.
> I don't have the literature how the plot gets constructed, but my
> understanding from
> http://www.itl.nist.gov/div898/software/dataplot/refman1/auxillar/meandiff.htm
> 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.
>
> May thanks
>
> Frank
>
> --
> 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
> London

```