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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