[R] Within-subject factors in lme
Kim Mouridsen
kim.mouridsen at gmail.com
Fri Jan 12 18:25:28 CET 2007
Dear Thilo
Thanks for your suggestion. I guess the model you are fitting here has
only a single random effect term, namely subject. If the effect of A
depends on S, one needs to include an additional random effects term
for the S:A interaction.
With lme I can get output for the effect of A which is very similar to
the aov output using
lme( y ~ A + B, random=~ 1|S/A )
but here I have cheated by not including factor B in the 'random='
terms. But the output from
anova( lme( y ~ A + B, random=~ 1|S/A ) )
is
numDF denDF F-value p-value
(Intercept) 1 54 388.4006 <.0001
B 2 54 154.0193 <.0001
A 1 13 4.4581 0.0547
where the last line appears equivalent to the aov output:
Error: Subject:Treatment
Df Sum Sq Mean Sq F value Pr(>F)
A 1 0.66074 0.66074 4.4581 0.05467 .
Residuals 13 1.92676 0.14821
But I still need to account for the random S:B interaction.
I can see a similar issue has been discussed earlier, see eg
https://stat.ethz.ch/pipermail/r-help/2006-August/111018.html
Here, lme( y ~ A*B, random=~1|S ) was also suggested (essentially),
but this gives quite different results from aov and the lme example
above. In this particular case I get
numDF denDF F-value p-value
(Intercept) 1 67 388.3976 <.0001
B 2 67 104.8436 <.0001
A 1 67 10.3707 0.002
I have seen instances of something like
random=list(S=pdBlocked(list(pdIdent(~A-1)..., but I can't get this to
work (and I have no idea what this does).
Best regards,
Kim.
2007/1/12, Thilo Kellermann <tkellermann at ukaachen.de>:
> Dear Kim,
> as far as I understandyour problem correct the specification of the model in
> lme is:
>
> lme( fixed=y ~ A*B, random=~1|S)
>
> Thilo
>
> On Friday 12 January 2007 15:54, Kim Mouridsen wrote:
> > Dear R-users
> >
> > I'm considering a repeated measures experiment where two
> > within-subject factors A (2 levels) and B (3 levels) have been
> > measured for each of 14 subjects, S. I wish to test the effect of
> > factor A. I know that a variance component model with random effects
> > S, S:A, S:B and S:A:B can be fitted using aov:
> >
> > aov( y ~ A*B + Error(S/(A*B)) )
> >
> > If there is no significant interaction, the test for the effect of A
> > is carried out in the S:A error strata.
> >
> > How can a test for the effect of A be performed using lme from the nlme
> > package?
> >
> > ( lme( y ~ A*B, random=~1|S/(A*B)) is apparently not correct )
> >
> > Thanks in advance for your advice.
> > Kim.
> >
> > ______________________________________________
> > R-help at stat.math.ethz.ch mailing list
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> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html and provide commented, minimal,
> > self-contained, reproducible code.
>
> --
> ________________________
> Thilo Kellermann
> Department of Psychiatry und Psychotherapy
> RWTH Aachen University
> Pauwelstr. 30
> 52074 Aachen
> Tel.: +49 (0)241 / 8089977
> Fax.: +49 (0)241 / 8082401
> E-Mail: tkellermann at ukaachen.de
>
>
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