[R] setting value arg of pdSymm() in nlme
William Valdar
valdar at well.ox.ac.uk
Tue May 17 16:22:07 CEST 2005
Hi,
> I'm afraid that I don't understand what you are trying to do. With a
> formula of ~ 1 the pdSymm generator creates a 1x1 variance-covariance
> matrix, which you are initializing to a 3x3 matrix.
Oh... I had a feeling I was doing something wrong there.
> What is batch.mat supposed to represent?
I would like to use batch.mat to specify a correlation structure for the
batches A, B and C. Specifically, I wish to work out the contribution to
the variance of the batch random effect, given that I know some pairs of
batches (eg, A and B) are going to be more similar than other pairs (eg, A
and C) and how similar they are likely to be. My (mis?)reading of P&B p165
suggested this may be possible turning some of the pdIdents into
pdSymms.
I was hoping to use this test example as a prelude to using genetic
relationship data to impose a correlation structure on a subject-level or
family-level effect. I understand from an earlier post (Jarrod Hadfield,
2003) that lme is not really optimized for this, but I would nontheless
like to evaluate to what extent it can do it anyway. The example used in
that post was extreme: each case was effectively a different realization
of a random effect where the correlation between levels is known. My needs
are simpler: in the example I gave above, batches A, B and C might
represent three families related by different degrees.
> Yes, although I think you mean lmer in lme4. Because the lmer function
> allows multiple nested or non-nested grouping factors, the need for the
> pdMat classes is eliminated (or greatly reduced) and the code can be
> simplified considerably. There is an article in the 2005/1 issue of R
> News describing the use of lmer.
Thanks for pointing this out as I had missed this article. I know lmer is
under development and am very interested in what it can do. Having found
lme/lmer useful for more standard problems I would like to use it for
genetic-type analysis where possible rather than resort to a different
language (eg SAS) or specialized proprietary software (eg, ASREML).
However, I understand that may not be possible.
William
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Dr William Valdar ++44 (0)1865 287 717
Wellcome Trust Centre valdar at well.ox.ac.uk
for Human Genetics, Oxford www.well.ox.ac.uk/~valdar
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