title: TukeyHSD or glht in R, ANCOVA
body: <p>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:</p>
  
  <pre><code>library(multcomp)
  data('litter', package = 'multcomp')
  litter.aov &lt;- aov(weight ~ gesttime + dose, data = litter)
  TukeyHSD(litter.aov, which = 'dose')
  </code></pre>
  
  <p>and i get a warning message like this:
  Warning message:
  In replications(paste("~", xx), data = mf): non-factor ignored: gesttime</p>
  
  <p>Is this process above correct? What's the meaning of the warning message? And does "TukeyHSD" apply to badly unbalanced designs?</p>
  
  <p>In addition, is there any difference between the processes above and below?</p>
  
  <pre><code>litter.mc &lt;- glht(litter.aov, linfct = mcp(dose = 'Tukey'))
  summary(litter.mc)
  </code></pre>
  
  <p>Best, Sue</p>