[R] Difference in ANOVA results - R vs. JMP/Minitab
Nirmal Govind
nirmalg at psu.edu
Thu Nov 20 05:49:27 CET 2003
Thanks for your reply John.
> this works). Applied to a linear-model object, summary() produces
> coefficients, etc. (as mentioned), while anova() produces a (sequential)
> ANOVA table. This seems apparent to me from the output.
What I'm having trouble with is understanding the difference between
aov() and lm() [since it seems like if I do a summary() after fitting
using aov(), the output is the same as doing anova() after an lm()].
Now, the outputs from aov() and lm() are different - the siginificant
effects are different. I think this may have to do with how these
functions treat the data - i.e. whether the function considers the data
as being in coded or uncoded units. Is this correct? From what I could
tell, aov() will code the data automatically and then present the ANOVA
table whereas lm() does not code the data. This pretty much explains
everything so far..
There's one problem though - how do I get the coefficients that are
calculated from the data after they are coded by aov()? The problem here
is that my factor levels are 0 and 1 instead of the usual -1 and 1...
if I run coefficients() after a aov() fit or an lm() fit, I get the same
coefficients... these coeffs. don't seem right (I compared with the
coefficients from Minitab and JMP -both give coefficients after coding
the data into a -1, 1 form).. I could of course modify my data and
change all the 0 levels to -1 but is there a way in R to get
coefficients that correspond to coded data?
> More generally, it probably makes sense to read introductory material
> about R -- such as the introductory manual that comes with the software
Yes, I have read some of these (and maybe I should read more :-))...
thanks for the pointer.. on the same note, is there any reference that
talks about how lm() and aov() treat data - coded vs. uncoded etc...
Thanks a lot for the help.. it is greatly appreciated!
nirmal
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