[R] Indexing ANOVA table

[Prof Brian Ripley]

On Sat, 29 Nov 2003 Pascal.Niklaus at unibas.ch wrote:

> Hi all,
> 
> I'd like to extract a value from an ANOVA table, but experience the following
> problem:
> 
> ### This works:
> 
> > s.pseudo <- summary(aov(m ~ block + mix*graz,data=split1))
> > s.pseudo
>             Df  Sum Sq Mean Sq F value  Pr(>F)
> block        2 1114.66  557.33  4.4296 0.04192 *
> mix          1    6.14    6.14  0.0488 0.82956
> graz         2    1.45    0.72  0.0057 0.99427
> mix:graz     2    3.82    1.91  0.0152 0.98495
> Residuals   10 1258.19  125.82
> ---
> Signif. codes:  0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
> > s.pseudo[[1]]["mix ","Pr(>F)"]
> [1] 0.8295556
> 
> ### But this doesn't -- why?

Why should it?

> > s.split <-  summary(aov(m ~ block + mix*graz + Error(Plot),data=split1))
> > s.split
> 
> Error: Plot
>           Df  Sum Sq Mean Sq F value Pr(>F)
> block      2 1114.66  557.33  0.8994 0.5265
> mix        1    6.14    6.14  0.0099 0.9298
> Residuals  2 1239.37  619.68
> 
> Error: Within
>           Df  Sum Sq Mean Sq F value Pr(>F)
> graz       2  1.4464  0.7232  0.3073 0.7437
> mix:graz   2  3.8206  1.9103  0.8117 0.4776
> Residuals  8 18.8278  2.3535
> > s.split[["Error: Plot"]]  ## extracting first list element works
>           Df  Sum Sq Mean Sq F value Pr(>F)
> block      2 1114.66  557.33  0.8994 0.5265
> mix        1    6.14    6.14  0.0099 0.9298
> Residuals  2 1239.37  619.68
> > s.split[["Error: Plot"]]["mix ","Pr(>F)"]  ### <== FAILS
> Error in s.split[["Error: Plot"]]["mix ", "Pr(>F)"] :
>         incorrect number of dimensions

You should have written

s.split[["Error: Plot"]][[1]]["mix ","Pr(>F)"]

> So where is the difference between the two?

One is an "aov" object, the other an "aovlist" object.  Take a closer look 
at print.summary.aov, for example.

More generally, learn how to look at R objects instead of assuming you
know what you are doing: unclass(s.split[["Error: Plot"]]) would have been
informative.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595