On Wed, 19 Nov 2003, Nirmal Govind wrote: > 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..
And all of that is explained in the reference on the aov help page. Please do stop speculating and start reading. > 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? AoV has nothing to do with coefficients. There is dummy.coef to get uncoded coefficients, and there is options(contrasts=) to set the coding. > > 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... Yes, you should read a lot more. Chapter 6 of Venables & Ripley (2002) and chapter 5 of Chambers & Hastie (1991) would be a good start. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
