Grokbase Groups R r-help March 2011
FAQ
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 at agrsci.dk

I use R.2.12.2, package version of multcomp : 1.2-5

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  • Richard M. Heiberger at Mar 4, 2011 at 7:33 pm
    Use backquotes

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



    On Fri, Mar 4, 2011 at 2:03 PM, Ulrich Halekoh 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

    ______________________________________________
    r-help@r-project.org mailing list
    https://stat.ethz.ch/mailman/listinfo/r-help
    PLEASE do read the posting guide
    http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
    and provide commented, minimal, self-contained, reproducible code.
  • William Dunlap at Mar 4, 2011 at 7:33 pm

    -----Original Message-----
    From: r-help-bounces at r-project.org
    [mailto:r-help-bounces at r-project.org] On Behalf Of Ulrich Halekoh
    Sent: Friday, March 04, 2011 11:04 AM
    To: r-help at r-project.org
    Subject: [R] glht: Problem with symbolic contrast for factors
    withnumber-levels

    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
    Try putting backquotes around the factor levels so
    the R parser makes names out of them.
    glht(motA,linfct=mcp(A=c("`2` - `1` = 0 ")))
    General Linear Hypotheses

    Multiple Comparisons of Means: User-defined Contrasts


    Linear Hypotheses:
    Estimate
    `2` - `1` == 0 2

    Bill Dunlap
    Spotfire, TIBCO Software
    wdunlap tibco.com
    #works
    glht(motB,linfct=mcp(B=c("f - e = 0")))

    regards

    Ulrich Halekoh
    Aarhus University
    e-mail: Ulrich.Halekoh at agrsci.dk

    I use R.2.12.2, package version of multcomp : 1.2-5

    ______________________________________________
    R-help at r-project.org mailing list
    https://stat.ethz.ch/mailman/listinfo/r-help
    PLEASE do read the posting guide
    http://www.R-project.org/posting-guide.html
    and provide commented, minimal, self-contained, reproducible code.

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