[R] Random effects ANOVA?

[Peter Dalgaard]

"Chris Bergstresser" <chris at subtlety.com> writes:

> On 3/30/06, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
> > I think you want print or summary rather than anova.  anova() is not very
> > useful for aov() models even without error strata.
> 
>     That's sort of better.  summary(aov(time ~ drink + Error(video),
> data = df));
> 
> gives me:
> 
> Error: video
>           Df Sum Sq Mean Sq F value Pr(>F)
> Residuals  2    160      80
> 
> Error: Within
>           Df  Sum Sq Mean Sq F value    Pr(>F)
> drink      1 240.000 240.000  44.211 1.313e-08 ***
> Residuals 56 304.000   5.429
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> 
> ...what I'm really looking for is something akin to the output from
> SAS, which is:
> 
> Source     DF     Anova SS    Mean Square   F Value    Pr > F
> DRINK       1      240.000        240.000     12.00    0.0742
> VIDEO       2      160.000         80.000      4.00    0.2000
> 
> > I didn't follow how the videos were chosen.  Random effects apply when the
> > 'treatments' were chosen from a large population (which might apply if
> > each subject watched (on separate occasions) three videos chosen randomly
> > from a larger pool), and if the interest is in the variability of the
> > response over videos in the pool.  If subjects were observed more than
> > once then I suspect you most likely want a random effect for subjects.
> 
>    This problem comes directly from the final for my Experimental
> Stats class, which is why it feels a little odd.  The videos were
> randomly selected from a library.  Subjects watched one of the three
> videos, drank one of the two drinks, and completed the tasks.  There
> were no repeated measures, so we can't block on subjects.
>    The hypothesis test, according to SAS, treats DRINK*VIDEO as an
> error term.  Setting aside whether this is the right analysis, how can
> I replicate this analysis in R?

Possibly,

  time ~ drink + Error(video/drink)

It won't give you the F test for "video" though. Not quite sure
whether

  time ~ drink + video + Error(video:drink)

works. It might, although it is a bit unnatural to have a random
interaction between to systematic effects.


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
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