[R] multiple comparisons with generalised least squares

[Sandy Adriaenssens]

Dear R users,

I have used the following model:

M1 <- gls(Nblad ~ Concentration+Season + Concentration:Season, data=DDD,
weights=varIdent(form=~ 1 | Season*Concentration))

to assess the effect of Concentration and Season on nitrogen uptake by
leaves (Nblad). I accounted for the difference in variance across the factor
levels by using the varIdent function.

Then I wanted to perform multiple comparisons with the glht function of the
multcomp package.

glht(M1,linfct = mcp(Season = "Tukey"))

However, here I got an error message "Error in terms.default(object) : no
terms component. Error in factor_contrasts(model) : no ‘model.matrix’ method
for ‘model’ found!".

Does the glht function work with a gls model? And if not, is there an other
way to perform multiple comparisons for a gls model?

I've searched this forum for an answer to this question, but I could only
found someone with the same question which remained unanswered. I hope
someone can provide an answer now!

Many thanks in advance!

Sandy



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