[R] repeated measures help; disagreement with SPSS

Greg Trafton trafton at itd.nrl.navy.mil
Wed Oct 9 03:18:33 CEST 2002

Peter Dalgaard BSA <p.dalgaard at biostat.ku.dk> writes:

> Argh. You have no idea how difficult it is to read that stuff when
> you're not on a Windows machine... But I suppose that converting it to
> plain text is a pain even *on* Windows.

My apologies.  I hate the program and environment too, which is one of
the reasons I'm trying to change ;-)

> Anyways, as far as I can see, you are in fact getting the same
> interaction test (RL*COND, Sphericity Assumed, Type III SS=17.734), so
> I'd suspect that the test for the main effect is one of those weird
> things where  you take the average over the three levels of cond,
> ignoring the fact that one level occurs twice as often as the others.
> What happens if you run the SPSS analysis without the interaction term? 

YES!  You were exactly right.  I tried to change the model in SPSS,
but it wouldn't let me in that case (it always includes the
interaction, grumble).  so I made every condition have the same
number of data points.  and that showed up as the exact same set of
numbers in SPSS and R (whew!):  same p values, same SS, MS, etc.

OK, so the next question is, I understand your reasoning about the
weird thing, but which one *should* I use, the weighted version (like
R) or the unweighted (like SPSS)?  (realistically, this probably
doesn't happen too much, and certainly not to this degree, so it
probably doesn't have much of a profound effect in any case).  This is
probably not the right place for this question, but ...

thanks a ton for your help!

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