Paul Gribble wrote:
I have 3 questions (below).
Background: I am teaching an introductory statistics course in which we are
covering (among other things) repeated measures anova. This time around
teaching it, we are using R for all of our computations. We are starting by
covering the univariate approach to repeated measures anova.
Doing a basic repeated measures anova (univariate approach) using aov()
seems straightforward (e.g.:
+> myModel<-aov(myDV~myFactor+Error(Subjects/myFactor),data=myData)
+> summary(myModel)
Where I am currently stuck is how best to deal with the issue of the
assumption of homogeneity of treatment differences (in other words, the
sphericity assumption) - both how to test it in R and how to compute
corrected df for the F-test if the assumption is violated.
Back when I taught this course using SPSS it was relatively straightforward
- we would look at Mauchly's test of sphericity - if it was significant,
then we would use one of the corrected F-tests (e.g. Greenhouse-Geisser or
Huynh-Feldt) that were spat out automagically by SPSS.
I gather from searching the r-help archives, searching google, and searching
through various books on R, that the only way of using mauchly.test() in R
is on a multivariate model object (e.g. mauchly.test cannot handle an aov()
object).
Question 1: how do you (if you do so), test for sphericity in a repeated
measures anova using R, when using aov()? (or do you test the sphericity
assumption using a different method)?
Question 2: Can someone point me to an example (on the web, in a book,
wherever) showing how to perform a repeated measures anova using the
multivariate approach in R?
Question 3: Are there any existing R functions for calculating adjusted df
for Greenhouse-Geisser, Huynh-Feldt (or calculating epsilon), or is it up to
me to write my own function?
Thanks in advance for any suggestions,
Have a look at
http://cran.r-project.org/doc/Rnews/Rnews_2007-2.pdf
Last time this came up, John Fox also pointed to some of his stuff, see
http://finzi.psych.upenn.edu/R/Rhelp08/archive/151282.html
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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