[R] Pairwise comparison, TukeyHSD, glht, ANCOVA

[Wang ChenXi]

Hi,

I'm wondering if i can use the function "TukeyHSD" to perform the all pairwise comparisons of a "aov()" model with one factor (e.g., GROUP) and one continuous covariate (e.g., AGE). I did for example:

library(multcomp)
data('litter', package = 'multcomp')
litter.aov <- aov(weight ~ gesttime + dose, data = litter)
TukeyHSD(litter.aov, which = 'dose')

and i get a warning message like this:
Warning message:
In replications(paste("~", xx), data = mf): non-factor ignored: gesttime

Is this process above correct? What's the meaning of the warning message? And does "TukeyHSD" apply to badly unbalanced designs?

In addition, is there any difference between the processes above and below?

litter.mc <- glht(litter.aov, linfct = mcp(dose = 'Tukey'))
summary(litter.mc)

Sincerely,
Sue




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