[R] Fixed and random factors in aov()
Sven Garbade
garbade at mip.paed.uni-muenchen.de
Fri Jul 16 10:36:53 CEST 2004
Hi,
I'm confused about how to specify random and fixed factors in an aov()
term. I tried to reproduce a textbook example: One fixed factor (Game, 4
levels) and one random factor (Store, 12 levels), response is Points.
The random factor Store is nested in Game. I tried
> str(kh.df)
`data.frame': 48 obs. of 4 variables:
$ Subj : Factor w/ 48 levels "vp1","vp10","vp11",..: 1 12 23 34 44...
$ Game : Factor w/ 4 levels "S1","S2","S3",..: 1 1 1 1 1 1 1 1 ...
$ Store : Factor w/ 12 levels "KH1","KH10","KH11",..: 1 1 1 1 5 ...
$ Points: num 7 9 12 7 6 5 8 6 9 6 ...
> summary(aov(Points ~ Game + Error(Game:Store),dat=kh.df))
Error: Game:Store
Df Sum Sq Mean Sq F value Pr(>F)
Game 3 391.50 130.50 5.9771 0.01934 *
Residuals 8 174.67 21.83
---
Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
Error: Within
Df Sum Sq Mean Sq F value Pr(>F)
Residuals 36 149.500 4.153
Warning message:
Error model is singular in: aov(Points ~ Game + Error(Game:Store), dat =
kh.df)
which gives me the correct F-statistic for the fixed factor Game and
> summary(aov(Points ~ Game + Game:Store,dat=kh.df))
Df Sum Sq Mean Sq F value Pr(>F)
Game 3 391.50 130.50 31.4247 3.707e-10 ***
Game:Store 8 174.67 21.83 5.2575 0.0002126 ***
Residuals 36 149.50 4.15
---
Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
which calcultes the correct Game:Store F statistic but the incorrect
F value for Game (because its a fixed factor). I'm wondering about how
to specify a correct model formula for the fixed and random factors with
an appropiate Error() term.
Thanks, Sven
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