On 5/9/07, shabnam shademan <[EMAIL PROTECTED]> 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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