Hi all,
I have been trying to calculate Type III SS in R for an unbalanced two-way
anova. However, the Type III SS are lower for the first factor compared to
type I but higher for the second factor (see below). I have the impression
that Type III are always lower than Type I - is that right?
And a clarification about how to fit Type III SS. Fitting model<-aov(y~a*b)
in the base package and then loading car / changing contrasts / running
Anova(model,type=c("III")) gives different results compared to loading car /
changing contrasts / fitting model<-aov(y~a*b) / running
Anova(model,type=c("III")). However summary(model) gives the same results in
both cases. Is this how it is set up?
> local({pkg <- select.list(sort(.packages(all.available = TRUE)))
+ if(nchar(pkg)) library(pkg, character.only=TRUE)})
> options(contrasts=c("contr.helmert","contr.poly"))
> model2<-aov(tdrate~temp*sex)
> summary(model2)
Df Sum Sq Mean Sq F value Pr(>F)
temp 3 0.110137 0.036712 1005.6947 < 2e-16 ***
sex 1 0.000141 0.000141 3.8593 0.05095 .
temp:sex 3 0.000154 0.000051 1.4073 0.24206
Residuals 187 0.006826 0.000037
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> Anova(model2,type=c"III")
Error: unexpected string constant in "Anova(model2,type=c"III""
> Anova(model2,type=c("III"))
Anova Table (Type III tests)
Response: tdrate
Sum Sq Df F value Pr(>F)
(Intercept) 0.57549 1 15764.9249 < 2e-16 ***
temp 0.08571 3 782.6314 < 2e-16 ***
sex 0.00023 1 6.2851 0.01303 *
temp:sex 0.00015 3 1.4073 0.24206
Residuals 0.00683 187
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
--
Menelaos Stavrinides
Ph.D. Candidate
Environmental Science, Policy and Management
137 Mulford Hall MC #3114
University of California
Berkeley, CA 94720-3114 USA
Tel: 510 717 5249
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