[R] two way ANOVA with unequal sample sizes
julien claude
claude at isem.univ-montp2.fr
Tue Oct 16 21:33:48 CEST 2001
Hi,
I am trying a two way anova with unequal sample sizes but results are not
as expected:
I take the example from Applied Linear Statistical Models (Neter et al.
pp889-897, 1996)
growth rate gender bone development
1.4 1 1
2.4 1 1
2.2 1 1
2.4 1 2
2.1 2 1
1.7 2 1
2.5 2 2
1.8 2 2
2 2 2
0.7 3 1
1.1 3 1
0.5 3 2
0.9 3 2
1.3 3 2
expected results are
source of variation SS df MS F
gender 0.12 1 0.12 0.74
bone development 4.1897 2 2.0949 12.89**
interaction 0.0754 2 0.377 0.23
Error 1.3 8 0.1625
# I use
aov (growrate ~ gender * bonedevelopment)->m
summary(m)
Df Sum Sq Mean Sq F value Pr(>F)
as.factor(gender) 2 4.3063 2.1531 13.2501
0.002891 **
as.factor(bonedevlopment) 1 0.0926 0.0926 0.5697
0.472022
as.factor(gender:bonedevlopment) 2 0.0754 0.0377 0.2321 0.798034
Residuals 8 1.3000 0.1625
#if I change the order of factors, results are different
aov (growrate ~ bonedevelopment * gender)->m
summary(m)
Df Sum Sq Mean Sq F value
Pr(>F)
as.factor(bonedevlopment) 1 0.0029 0.0029 0.0176
0.897785
as.factor(gender) 2 4.3960 2.1980 13.5262 0.002713 **
as.factor(gender:bonedevlopment) 2 0.0754 0.0377 0.2321 0.798034
Residuals 8 1.3000 0.1625
#In the both cases, results for main effects differ from those expected in
Neter et al.
However interaction and residuals are well estimated.
Can anyone help, either I am wrong in the formula, or either is there an
other problem? Is there a mean to conduct easily the test as in it is in
Neter et al. ?
The same problems occurs with anova(lm(....))?
thank you very much
julien CLAUDE
-------------------------------
CLAUDE julien
Université Montpellier II
Institut des Sciences de l'Evolution de Montpellier
Laboratoire de Paléontologie (Morphométrie), Cc64
2, Place Eugène Bataillon.
34095, Montpellier, cedex 5
FRANCE
Phone : (33) 4 67 14 47 82
Fax : (33) 4 67 14 36 10
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