Hello all,
A really noddy question for you all: I'm trying without success to do some
subhypothesis testing. Using simple anova model, with a toy dataset from a book. I
have four factors A,B,C,D, and wish to test mu_C = mu_D. This is what I have tried:
> contrasts(infants$group,how.many=1) <- c(0,0,1,-1)
> contrasts(infants$group)
[,1]
A 0
B 0
C 1
D -1
> fit <- aov(age~group,data=infants)
> summary(fit)
Df Sum Sq Mean Sq F value Pr(>F)
group 1 0.740 0.740 0.2693 0.6092
Residuals 21 57.727 2.749
Now I know from the book, hand calculations and SPSS that for "group", Sum Sq = Mean
Sq = 1.12, not 0.740. Also from the standard anova:
> contrasts(infants$group) <- "contr.treatment"
> fit <- aov(age~group,data=infants)
> summary(fit)
Df Sum Sq Mean Sq F value Pr(>F)
group 3 14.778 4.926 2.1422 0.1285
Residuals 19 43.690 2.299
So I'd like to have the (correct) Mean Sq value divided by 2.299 and not 2.749, with
19 and not 21 df. Any advice on how to correctly use contrasts for subhypothesis
testing, including where to find it in the manuals, would be much appreciated.
Cheers,
Rob.
(Other info)
Using R 1.6.2, windows xp,
data:
> infants
group age
1 A 9.00
2 A 9.50
3 A 9.75
4 A 10.00
5 A 13.00
6 A 9.50
7 B 11.00
8 B 10.00
9 B 10.00
10 B 11.75
11 B 10.50
12 B 15.00
13 C 11.50
14 C 12.00
15 C 9.00
16 C 11.50
17 C 13.25
18 C 13.00
19 D 13.25
20 D 11.50
21 D 12.00
22 D 13.50
23 D 11.50
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