On 11/04/07 16:34, Gilbert G wrote: > Dear R people: > > I wish to switch from SPSS to R, but there is one particular type of > ANOVA design that cannot be done in R. Or more likely, it can be > done, but it is nowhere documented. > > The problem is typical for psychologists: > You have a repeated measures design with different groups of subjects. > Now, this can be done with the aov command, but the number of > subjects in both groups must be equal (i.e., balanced design). SPSS > allows for unbalanced designs as well. > > If you are still with me, let me just give you an example of what R > can and cannot do so far. Imagine I have a 2x2 within subjects design > and I have 2 groups (e.g., group healthy and patients, which is stored > in MyGroup). And imagine I measure reaction time RT in four > conditions, say, in a color condition (red vs green) and in a shape > condition (square vs circle).
At the risk of getting in trouble, let me suggest another approach. Compute the relevant terms for each subject, then do a t test comparing your two groups. The t test does not assume equal sized groups. Yuelin Li, in our "Notes on the use of R ..." shows how to use a t test to check a design very similar to what you suggest: http://www.psych.upenn.edu/~baron/rpsych/rpsych.html (section 6.10, I think, perhaps elsewhere too). You can do this either with a loop or with the lmList() function in the lme4 package (which is not discussed yet in our "Notes..."). For example, with a loop, you would compute CS[i] <- RTredsquare[i] - RTbluesquare[i] - RTredcircle[i] + RTbluecircle[i] and then do t.test(CS ~ Group) to see if your two groups differ in the interaction effect. It is easier with lmList. You don't need the loop. I do think that nlme is going to replace a lot of standard approaches in psychology. (I am almost to the point of understanding it.) But I don't think it is necessary for the kind of design you describe. Jon -- Jonathan Baron, Professor of Psychology, University of Pennsylvania Home page: http://www.sas.upenn.edu/~baron ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
