[R] (REPOST) Simple main effects in 2-way repeated measure ANOVA
gpagnon at emory.edu
Mon Aug 9 17:19:26 CEST 2004
Dear Prof. Ripley
I am sorry that reposting the mail was perceived as not complying to the
correct list etiquette: I do not usually frequent the list and I just
thought that the message went unnoticed. I apologize for the insistence.
On the subject matter of the e-mail, yes, there was a typo, the error
term was indeed Error(Subj/Time). But I am still interested in
examining the simple main effects, when the interaction of Time and
Group is significant:
ControlGroup: before vs after
TreatmentGroup: before vs after
Before: ControlGroup vs TreatmentGroup
After: ControlGroup vs TreatmentGroup
Is this incorrect statistical praxis? Other software packages display
the results for the above tests after the ANOVA. However, I couldn't
find a simple way to extract this information in R.
Also, I am not sure how to include a confounding variable, like Age, in
the formulation of the model.
Thanks for your help
Prof Brian Ripley wrote:
>Why has this been REPOSTed? It was delivered last Thursday.
>On Sun, 8 Aug 2004, Giuseppe Pagnoni wrote:
>>I am running a 2-way repeated measure anova with 1 between-subjects
>>factor (Group=treatment, control), and 1 within-subject factor (Time of
>>measurement: time1, time2). I extract the results of the anova with:
>>summary(aov(effect ~ Group*Time + Error=Subj/Time, data=mydata))
>That's not valid syntax for an R formula. Did you mean Error(Subj/Time)?
>>Now, this must be clearly a dumb question, but how can I quickly
>>extract in R all the post-hoc t-tests for the simple main effects?
>I very much hope you cannot, as you have specified an interaction, and
>you should not want t-tests for main effects in the presence of an
>interaction, and certainly not with the default R coding.
>Did you mean Group + Time?
>>Also, while I am at it, how do I enter in the model a counfounding
>>covariate (e.g., Age)?
>>And on a different matter, is there a way to receive interactive user
>>input in an R script? Something like "Enter the name of the factor:
>>", or even more simply "Press <Enter> to see the result of the next
>?readline, or cat + scan or ... using connections.
Giuseppe Pagnoni, Ph.D.
Dept. of Psychiatry and Behavioral Sciences
1639 Pierce Drive, Suite 4000
WMB Bldg., Atlanta, GA 30322, U.S.
e-mail: gpagnon at emory.edu
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