[R] TukeyHSD & Covariates
p.dalgaard at biostat.ku.dk
Wed Dec 15 22:54:51 CET 2004
DamiÃ¡n Cirelli <damian.cirelli at maine.edu> writes:
> Dear R gurus,
> I have the following model:
> appcov.aov <- aov(yield ~ prevyield + trt + block)
> where prevyield is a continuous numeric covariate and trt and block
> are factors (yes, I did factor()!)
> Now, when I do a TukeyHSD, my diff's are all screwed up!
> For instance:
> treatment mean for treatmen "E" is 277.25 and for treatment "O" is
> 279.5, so I figure the diff O-E should be 2.25, but TukeyHSD says:
> diff lwr upr
> O-E -50.817101 -84.8112057 -16.822996
> So I wonder where is that -50.8 coming from???
> Anybody have a clue?
> Thanks a lot!
> PS: it works if I take prevyield (the covariate) out of the model, but
> the point is I need to analyse it with the covariate.
> Thanks again
If the covariate level differs between the treatment groups, then the
difference in the covariate-adjusted means could well differ quite a
bit from the unadjusted difference. What happens if you do
summary(lm(yield ~ prevyield + trt + block))
(Not sure I'm happy about using the HSD procedure with an unbalanced
O__ ---- Peter Dalgaard Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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