I am at my machine now. The subset of the data you sent has only two groups and doesn't have a significant interaction. Therefore I can't talk about specifics for your example.
The glht test you did compared the means of the sampling factor ignoring the dist covariate. Your description said you wanted to compare the slopes. I would start by looking at the slopes of the four sampling values in the graph. You can probably see on the graph what is happening since you have the full dataset available. calculating the glht comparisons will be much more difficult than looking at the plot. On Sun, Feb 12, 2012 at 3:11 PM, Rmh <r...@temple.edu> wrote: > ancova in HH is a wrapper for aov > that displays a set of lattice plots. > > the problem you are seeing is probably that glht ignores covariates (with > an appropriate message) unless you specify an optional argument. > I will reply in more detail when i am at > my computer. > > in the meantime, look at ?glht in the multcomp package and at > ?mmc in the HH package for examples. > > Sent from my iPhone > > On Feb 12, 2012, at 13:28, peter dalgaard <pda...@gmail.com> wrote: > > > Inline below > > > > On Feb 12, 2012, at 13:39 , Evagelopoulos Thanasis wrote: > > [...] > >> > >> Because there exist significantly different regression slopes, I did a > post hoc test with glht() to find out between which samplings: > >> > >>> summary(glht(mod, linfct=mcp(sampling="Tukey"))) > >> > > > > I believe this compares the intercepts, not slopes. Slope differences > are in the sampling:dist interaction terms. > > > >> The results seem to say that there are no significantly different > slopes for any of the pair-wise comparisons of factor levels: > >> > >> Simultaneous Tests for General Linear Hypotheses > >> > >> Multiple Comparisons of Means: Tukey Contrasts > >> > >> > >> Fit: aov(formula = h ~ sampling * dist, data = data) > >> > >> Linear Hypotheses: > >> Estimate Std. Error z value Pr(>|z|) > >> sp - au == 0 0.06696 0.04562 1.468 0.457 > >> su - au == 0 -0.02238 0.04562 -0.491 0.961 > >> wi - au == 0 0.01203 0.04562 0.264 0.994 > >> su - sp == 0 -0.08934 0.04562 -1.958 0.204 > >> wi - sp == 0 -0.05493 0.04562 -1.204 0.624 > >> wi - su == 0 0.03441 0.04562 0.754 0.875 > >> (Adjusted p values reported -- single-step method) > >> > > > > We don't have coefficients for your model, so it is a bit hard to tell > what the parameter functions are, but I would expect those NOT to be the > slope differences. > > > >> Warning message: > >> In mcp2matrix(model, linfct = linfct) : > >> covariate interactions found -- default contrast might be inappropriate > >> > >> > >> > >> My questions are: > >> > >> - Did I make a mistake somewhere? (I probably did!) > > > > You need to figure out how to get glht to look at the appropriate linear > hypothesis (and mcp(sampling=...) is not right). I'd do a straight lm() > analysis so that I'd know exactly what the parameters mean -- aov() can be > a little too good at hiding technical details from the user! > > > >> - Could I do pairwise ANCOVAs and thus have just two factor levels > (=two regression slopes) to compare each time? > > > > Possibly, but you'd lose the multiple comparison features of glht. > > > >> What does the warning message "covariate interactions found -- default > contrast might be inappropriate" mean? > >> > > > > That you likely don't want to look at intercepts (or whatever the > "sampling" parameters represent --- I'm not familiar with that ancova() > function) in the presence of interactions... > > > >> Thank you! > >> Athanasios Evagelopoulos > >> ______________________________________________ > >> 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. > > > > -- > > Peter Dalgaard, Professor, > > Center for Statistics, Copenhagen Business School > > Solbjerg Plads 3, 2000 Frederiksberg, Denmark > > Phone: (+45)38153501 > > Email: pd....@cbs.dk Priv: pda...@gmail.com > > > > ______________________________________________ > > 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. > [[alternative HTML version deleted]] ______________________________________________ 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 and provide commented, minimal, self-contained, reproducible code.
