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confint(glht(motA,linfct=mcp(A=c("`2` - `1` = 0 "))))




On Fri, Mar 4, 2011 at 2:03 PM, Ulrich Halekoh <Ulrich.Halekoh@agrsci.dk>wrote:

> Using a factor with  'number'  levels the straightforward
> symbolic formulation of a contrast in 'glht' of
> the 'multcomp' package fails.
>
> How can this problem be resolved without having to redefine the factor
> levels?
>
> Example:
> #A is a factor with  'number' levels
> #B similar factor with 'letter' levels
> dat<-data.frame(y=1:4,A=factor(c(1,1,2,2)),
>                      B=factor(c('e','e','f','f')) )
> motA<-lm(y~A,data=dat)
> motB<-lm(y~B,data=dat)
> library(multcomp)
> #does not work
> glht(motA,linfct=mcp(A=c("2 - 1 = 0 ")))
>
> #the error message is
> # Error in coefs(ex[[3]]) :
> #   cannot interpret expression '1' as linear function
>
> #works
> glht(motB,linfct=mcp(B=c("f - e  = 0")))
>
> regards
>
> Ulrich Halekoh
> Aarhus University
> e-mail: Ulrich.Halekoh@agrsci.dk
>
> I use R.2.12.2, package version of multcomp : 1.2-5
>
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>

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