[R-lang] comparing two mixed-effects models

[T. Florian Jaeger]

On 5/9/07, shabnam shademan <shademan at ucla.edu> wrote:
>
>       3. could I use the same formula to examine only one "Age" group
> (removing "Age" as a factor in the formula, of course), even if I am going
> to later proceed and re-examine the data for a larger young/old set?
>
>
> I am not sure that I understand what you mean. Nothing prevents you from
> exploring e.g. only data from "young" people, but be aware that whatever
> conclusions you draw out of the examination of that subset of your data, may
> not generalize to the entire sample (and hence not to the population
> represented by the entire sample).
>
> SS: the data shows an effect of "Age" (which has two levels "young" and
> "old"). I wanted to test a subset of the data that included only "young" and
> see whether there is an interaction between X and Y, for them as a subset.
> I was thinking that the result may be generalized to only "young"
> population, and not the entire sample.  Just want to make sure that I am not
> doing something that has problems that I am not aware of.  I hope I was able
> to state the question more clearly.
>
>
> yes, this is ok.  be aware, however, that by splitting the data set you
> loose power. so failure to detect the X*Y interaction MAY be due to lack of
> power. an additional test you could confirm is a three-way interaction for
> the entire data set X*Y*Age. If it is NON-significant that is a further
> indication that the X*Y interaction holds for both age groups.
>
> SS: Thanks.  The X*Y interaction does not disappear for the smaller set,
> and the three-way interaction Age*X*Y in not significant.  So, it looks like
> all is well.
>
>
>
>
>
> finally, I always advise visualization of the data set (as a way to
> compare the effects of X*Y for young vs. old people).
>
> SS:I am not sure what kind of visualization you are recommending for X*Y.
>

oh, just plots of the two factors against the dependent variable. e.g. you
could plot two barplots for X*Y against the dependent variable - one for
young and one for old people. you could also print the fitted (predicted)
effects. some models fit functions (e.g. lrm in Design) have implemented
plot functions, but lmer does not, i believe. you could look into Baayen's
languageR library.

florian

is this useful? I am cc-ing the language list, so that other people can
> correct me.
>
> Yes, it is.  Thanks for taking the time.
>
> -shabnam
>
>
>
>
>
> Florian
>
>
>
> Thanks very much.
>
> -shabnam
>
>
>
>
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