Here is some toy data to give an example. Imagine someone is test-riding
several bicycles to decide which to buy, and measures their average speed
during each test-ride. They want to assess the main effects and interaction
of brand and material on speed.

Speed    Size    Brand        Material
20         small    giant        steel
25.1      medium    giant        steel
25.2      medium  trek        aluminum
26         small    cannondale    carbon
19.9      large    giant        aluminum
21         medium    trek        carbon
30         medium    cannondale    carbon

This is the output I get:

> fit = aov(Speed~Brand*Material, data=myData)
> summary(fit)
            Df Sum Sq Mean Sq F value Pr(>F)
Brand        2 49.862  24.931  2.3738 0.2964
Material     2 13.502   6.751  0.6428 0.6087
Residuals    2 21.005  10.503


I suppose I am missing something very obvious here as to why the interaction
is not reported, but I don't see what it is.
Cheers,

Gabe


On Wed, Apr 8, 2009 at 1:34 PM, Patrick Connolly <p_connolly@slingshot.co.nz
> wrote:

> On Wed, 08-Apr-2009 at 12:59PM -0700, Gabriel Murray wrote:
>
> |> I've noticed with certain datasets that when I try to do an anova and
> test
> |> for main effects and interaction for two explanatory variables,
> sometimes
> |> the main effect results are given but not the interaction results. For
> |> example,
> |>
> |> ex1 = aov(Score ~ var1*var2, data=myData)
> |> summary(ex1)
>
> Could it be that there are no degrees of freedom left?
>
> That's about my best guess without enough information.
>
>
> |> ______________________________________________
> |> R-help@r-project.org mailing list
> |> https://stat.ethz.ch/mailman/listinfo/r-help
> |> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> |> and provide commented, minimal, self-contained, reproducible code.
>
>
> Nothing you sent is reproducible.
>
>
> --
> ~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.
>    ___    Patrick Connolly
>  {~._.~}                   Great minds discuss ideas
>  _( Y )_                 Average minds discuss events
> (:_~*~_:)                  Small minds discuss people
>  (_)-(_)                              ..... Eleanor Roosevelt
>
> ~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.
>

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