[R] anova subhypotheses

[kjetil brinchmann halvorsen]

On 6 Mar 2003 at 10:38, rob foxall (IFR) wrote:

You can use linear.hypothesis() from the
package car (on CRAN).

Kjetil Halvorsen

> 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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> 
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