Hi Laura, If you want ANOVA output, ask for it! A general strategy that almost always works in R is to fit 2 models, one without the term(s) you want to test, and one with. Then use the anova() function to test them. (models must be nested, and in the lmer() case you need to use REML = FALSE).
So, try something like this: m1 <- lmer(PTR ~ Test + Group + (1 | student), data=ptr) m2 <- lmer(PTR ~ Test * Group + (1 | student), data=ptr) anova(m1, m2) Best, Ista On Tue, Oct 12, 2010 at 11:59 PM, Laura Halderman <lk...@pitt.edu> wrote: > Hello. I am new to R and new to linear mixed effects modeling. I am trying > to model some data which has two factors. Each factor has three levels > rather than continuous data. Specifically, we measured speech at Test 1, > Test 2 and Test 3. We also had three groups of subjects: RepTP, RepNTP and > NoRepNTP. > > I am having a really hard time interpreting this data since all the examples > I have seen in the book I am using (Baayen, 2008) either have continuous > variables or factors with only two levels. What I find particularly > confusing are the interaction terms in the output. The output doesn't > present the full interaction (3 X 3) as I would expect with an ANOVA. Instead, it only presents an interaction term for one Test and one Group, presumably comparing it to the reference Test and reference Group. Therefore, it is hard to know what to do with the interactions that aren't significant. In the book, non-significant interactions are dropped from the model. However, in my model, I'm only ever seeing the 2 X 2 interactions, not the full 3 X 3 interaction, so it's not clear what I should do when only two levels of group and two levels of test interact but the third group doesn't. > > If anyone can assist me in interpreting the output, I would really appreciate > it. I may be trying to interpret it too much like an ANOVA where you would > be looking for main effects of Test (was there improvement from Test 1 to > Test 2), main effects of Group (was one of the Groups better than the other) > and the interactions of the two factors (did one Group improve more than > another Group from Test 1 to Test 2, for example). I guess another question > to pose here is, is it pointless to do an LME analysis with more than two > levels of a factor? Is it too much like trying to do an ANOVA? > Alternatively, it's possible that what I'm doing is acceptable, I'm just not > able to interpret it correctly. > > I have provided output from my model to hopefully illustrate my question. > I'm happy to provide additional information/output if someone is interested > in helping me with this problem. > > Thank you, > Laura > > Linear mixed model fit by REML > Formula: PTR ~ Test * Group + (1 | student) > Data: ptr > AIC BIC logLik deviance REMLdev > -625.7 -559.8 323.9 -706.5 -647.7 > Random effects: > Groups Name Variance Std.Dev. > student (Intercept) 0.0010119 0.03181 > Residual 0.0457782 0.21396 > Number of obs: 2952, groups: studentID, 20 > > Fixed effects: > Estimate Std. Error t value > (Intercept) 0.547962 0.016476 33.26 > Testtest2 -0.007263 0.015889 -0.46 > Testtest1 -0.050653 0.016305 -3.11 > GroupNoRepNTP 0.008065 0.022675 0.36 > GroupRepNTP -0.018314 0.025483 -0.72 > Testtest2:GroupNoRepNTP 0.006073 0.021936 0.28 > Testtest1:GroupNoRepNTP 0.013901 0.022613 0.61 > Testtest2:GroupRepNTP 0.046684 0.024995 1.87 > Testtest1:GroupRepNTP 0.039994 0.025181 1.59 > > Note: The reference level for Test is Test3. The reference level for Group > is RepTP. The interaction p value (after running pvals.fnc with the MCMC) > for Testtest2:GroupRepNTP is p = .062 which I'm willing to accept and > interpret since speech data with English Language Learners is particularly > variable. > ______________________________________________ > 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. > -- Ista Zahn Graduate student University of Rochester Department of Clinical and Social Psychology http://yourpsyche.org ______________________________________________ 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.
