[R] Finding Interaction and main effects contrasts for two-way ANOVA

[Thompson, David (MNR)]

 Dale,

Other than the first SAS contrast, does the following demonstrate what
your asking for?
> summary(twoway)
 material temp       voltage   
 1:12     50:12   Min.   : 20  
 2:12     65:12   1st Qu.: 70  
 3:12     80:12   Median :108  
                  Mean   :106  
                  3rd Qu.:142  
                  Max.   :188  
> contrasts(twoway$material)
  2 3
1 0 0
2 1 0
3 0 1
> contrasts(twoway$temp)
   65 80
50  0  0
65  1  0
80  0  1
> fit <- aov(voltage ~ material*temp, data=twoway)
> summary.aov(fit)
              Df Sum Sq Mean Sq F value  Pr(>F)    
material       2  10684    5342    7.91  0.0020 ** 
temp           2  39119   19559   28.97 1.9e-07 ***
material:temp  4   9614    2403    3.56  0.0186 *  
Residuals     27  18231     675                    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 


# setting (partial) contrasts
> contrasts(twoway$material) <- c(1,-1,0) # ignoring the second
available df
> contrasts(twoway$temp) <- c(0,1,-1) # ignoring the second available df
> contrasts(twoway$material)
  [,1]  [,2]
1    1 -0.41
2   -1 -0.41
3    0  0.82
> contrasts(twoway$temp)
   [,1]  [,2]
50    0 -0.82
65    1  0.41
80   -1  0.41

> summary.aov(fit, split=list(material=list('m1-m2'=1), temp=list('t50 -
t80'=1)))
                                 Df Sum Sq Mean Sq F value  Pr(>F)    
material                          2  10684    5342    7.91 0.00198 ** 
  material: m1-m2                 1   3800    3800    5.63 0.02506 *  
temp                              2  39119   19559   28.97 1.9e-07 ***
  temp: t50 - t80                 1  11310   11310   16.75 0.00035 ***
material:temp                     4   9614    2403    3.56 0.01861 *  
  material:temp: m1-m2.t50 - t80  1   4970    4970    7.36 0.01146 *  
Residuals                        27  18231     675                    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

# other examples of setting contrasts
# compare m1 vs m2 and m2 vs m3
> contrasts(twoway$material) <- matrix(c(1,-1,0,1,1,-2), nrow=3)
> contrasts(twoway$material)
  [,1] [,2]
1    1    0
2   -1    1
3    0   -1
# compare m1 vs m2 and m1+m2 vs m3
> contrasts(twoway$material) <- matrix(c(1,-1,0,1,1,-2), nrow=3)
> contrasts(twoway$material)
  [,1] [,2]
1    1    1
2   -1    1
3    0   -2

I'm not sure if 'summary.aov' is the only lm-family summary method with
the split argument.

DaveT.
*************************************
Silviculture Data Analyst
Ontario Forest Research Institute
Ontario Ministry of Natural Resources
david.john.thompson at ontario.ca
http://ofri.mnr.gov.on.ca
*************************************
>-----Original Message-----
>From: Steele [mailto:dale.w.steele at gmail.com] 
>Sent: March 6, 2008 09:08 PM
>To: r-help at stat.math.ethz.ch
>Subject: [R] Finding Interaction and main effects contrasts 
>for two-way ANOVA
>
>I've tried  without success to calculate interaction and main effects
>contrasts using R.  I've found the functions C(), contrasts(),
>se.contrasts() and fit.contrasts() in package gmodels.  Given the url
>for a small dataset and the two-way anova model below, I'd like to
>reproduce the results from appended SAS code.  Thanks.  --Dale.
>
>  ## the dataset (from Montgomery)
>twoway <- read.table("http://dsteele.veryspeedy.net/sta501/twoway.txt",
>col.names=c('material', 'temp','voltage'),colClasses=c('factor',
>'factor', 'numeric'))
>
>  ## the model
>fit <- aov(voltage ~ material*temp, data=twoway)
>
>/* SAS code */
>proc glm data=twoway;
>class material temp;
>model voltage = material temp material*temp;
>contrast '21-22-31+32' material*temp 0 0 0 1 -1 0 -1 1 0;
>estimate '21-22-31+32' material*temp 0 0 0 1 -1 0 -1 1 0;
>contrast 'material1-material2' material 1 -1 0;
>estimate 'material1-material2' material 1 -1 0;
>contrast 'temp50 - temp80' temp 1 0 -1;
>estimate 'temp50 - temp80' temp 1 0 -1;
>run;