[R] Unbalanced ANOVA in R?
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Fri Mar 8 07:24:27 CET 2002
On Fri, 8 Mar 2002, Fredrik Karlsson wrote:
> Hi all
> I'm trying to complete a textbook example originally designed for SPSS
> in R, and I therefore need to find out how to compute an unbalanced
> ANOVA in R.
> I did a search on the mailinglist archives an found a post by Prof.
> Ripley saying one should use the lme function for (among other things)
> unbalanced ANOVAs, but I have not been able to use this object.
> My code gives me an error.. Why is that ?
> > aov(lme(DELAY ~ DOSE + TRIALS,data=epinuneq))
> > Error in getGroups.data.frame(dataMix, groups) :
> > Invalid formula for groups
> Any ideas? How do I get an ANOVA computation that can handle uniqual
> sampe sizes in R?
I said use lme, *not* aov and lme. Note, though, that the advice applied
to multistrata unbalanced anova, and this appears to be a two-way layout in
one stratum. You can use aov for that. (You can't use lme unless there are
two or more strata.) Just be careful how you interpret the output: in SAS
parlance aov gives you `Type I' sums of squares.
The reason why this is more difficult in R is that R unlike SPSS does not
guess which of the many possible interpretations you mean. We would need
to know a lot more about the actual statistical problem to tell you what
would be a good analysis in R.
Maybe you should look at a textbook designed for use with S/R not SPSS?
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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