# [R] interaction contrasts

Kay Cichini Kay.Cichini at uibk.ac.at
Fri Oct 15 15:09:24 CEST 2010

```..by some (extensive) trial and error reordering the contrast matrix and the
reference level
i figured it out myself -
for anyone who might find this helpful searching for a similar contrast in
the future:
this should be the right one:

c2<-rbind("fac2-effect in A"=c(0,1,0,0,0,0,0,0),
"fac2-effect in B"=c(0,1,0,0,0,1,0,0),
"fac2-effect in C"=c(0,1,0,0,0,0,1,0),
"fac2-effect in D"=c(0,1,0,0,0,0,0,1),
"fac2-effect, A*B"=c(0,0,0,0,0,1,0,0),
"fac2-effect, A*C"=c(0,0,0,0,0,0,1,0),
"fac2-effect, A*D"=c(0,0,0,0,0,0,0,1),
"fac2-effect, B*C"=c(0,0,0,0,0,-1,1,0),
"fac2-effect, B*D"=c(0,0,0,0,0,-1,0,1),
"fac2-effect, C*D"=c(0,0,0,0,0,0,-1,1))

summary(glht(mod,c2))

Kay Cichini wrote:
>
> hello,
>
> i was shortly asking the list for help with some interaction contrasts
> (see below) for which
> i had to change the reference level of the model "on the fly" (i read a
> post that this is possible in
> multcomp).
>
> if someone has a clue how this is coded in multcomp; glht() - please point
> me there.
>
> yours,
> kay
>
>
> Kay Cichini wrote:
>>
>> hello list,
>>
>> i'd very much appreciate help with setting up the
>> contrast for a 2-factorial crossed design.
>>
>> here is a toy example:
>>
>> library(multcomp)
>>
>> dat<-data.frame(fac1=gl(4,8,labels=LETTERS[1:4]),
>>                 fac2=rep(c("I","II"),16),y=rnorm(32,1,1))
>>
>> mod<-lm(y~fac1*fac2,data=dat)
>>
>> ## the contrasts i'm interressted in:
>>
>> c1<-rbind("fac2-effect in A"=c(0,1,0,0,0,0,0,0),
>>           "fac2-effect in B"=c(0,1,0,0,0,1,0,0),
>>           "fac2-effect in C"=c(0,1,0,0,0,0,1,0),
>>           "fac2-effect in D"=c(0,1,0,0,0,0,0,1),
>> 	    "fac2-effect, A*B"=c(0,0,0,0,0,1,0,0),
>>           "fac2-effect, A*C"=c(0,0,0,0,0,0,1,0),
>>           "fac2-effect, A*D"=c(0,0,0,0,0,0,0,1))
>>
>> summary(glht(mod,c1))
>>
>> ## now i want to add the remaining combinations
>> ## "fac2, B*C"
>> ## "fac2, B*D"
>> ## "fac2, C*D"
>> ## to the simultanous tests to see whether the effects
>> ## of fac2 within the levels of fac1 differ between
>> ## each combination of the levels of fac1, or not ??
>>
>>
>> yours,
>> kay
>>
>>
>>
>
>

-----
------------------------
Kay Cichini