[R] repeated measures help; disagreement with SPSS
Peter Dalgaard BSA
p.dalgaard at biostat.ku.dk
Wed Oct 9 00:27:00 CEST 2002
Greg Trafton <trafton at itd.nrl.navy.mil> writes:
> Peter Dalgaard BSA <p.dalgaard at biostat.ku.dk> writes:
>
> > Greg Trafton <trafton at itd.nrl.navy.mil> writes:
> >
> >> > summary(aov(rl ~ cond * session + Error(subj/(session)), data=mig))
> >>
> >> Error: subj
> >> Df Sum Sq Mean Sq F value Pr(>F)
> >> cond 2 28.305 14.153 1.9916 0.2311
> >> Residuals 5 35.531 7.106
> >>
> >> Error: subj:session
> >> Df Sum Sq Mean Sq F value Pr(>F)
> >> session 2 4.4502 2.2251 2.9868 0.09616 .
> >> cond:session 4 17.7335 4.4334 5.9509 0.01024 *
> >> Residuals 10 7.4499 0.7450
> >> ---
> >> Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
> >>
> >> (I ran this one this way b/c of a similar example from Baron's "Notes
> >> for psychology experiments. Unfortunately, neither the session nor
> >> the interaction cond:session are the same as SPSS's output, though the
> >> degrees of freedom are correct in both, of course).
> >>
> >> I'm certainly able to believe that SPSS is wrong and R is right, but
> >> thought I'd check with this list to make sure I'm not doing something
> >> completely stupid...
> >
> > This is consistent with both lm() (using subj as a systematic effect)
> > and lme(), so I'd strongly suspect that SPSS is getting it wrong. What
> > does SPSS give?
>
> OK, now I get to show off my ignorance of SPSS ;-) Perhaps I'm using
> it wrong, arg.
>
> I'm including the spool file and a word copy of it (which looks pretty
> ugly):
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.
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?
--
O__ ---- Peter Dalgaard Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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