On Jul 8, 2012, at 4:11 AM, Suresh Krishna wrote:
Hi,
I am a very occasional user of R, and will be grateful for some
help in constructing a regression across groups.
Here is an example:
library(MASS)
attach(cats)
Sex[120:144]<-factor(TG) #Renaming some males to transgender, to
create 3 groups, male, female and transgender
That throws an error for me. (and I expected it to do so since I was
guessing the "TG" was not in the levels of cats$Sex.) The error
actually had nothing to do with the lack of a "TG" level, but was
instead due to the lack of a TG object in my workspace. Didn't you get
an error?
If a level does not exist then you need to add it before you make
assignments to a facotr variable:
> levels(Sex) <- c(levels(Sex), "TG")
> Sex[120:144]<-"TG"
> table(Sex)
Sex
F M TG
47 72 25
out<-lm(Bwt~Sex/Hwt) #Gives me 3 separate linear regressions for
groups M, F and TG
What I now want to do is to examine subsets of this, where groups F
and TG have the same slope (and/or intercept). I want to do this in
such a way that future calls to anova() for model comparison will
recognize the subset structure: so I do not want to just reset the
levels with F to TG and then examine the regression.
I'm getting the sense that this is homework. You offer no information
about you business or academic affiliations and appear not to have
read the Posting Guide. Also setting up regressions is covered in the
"Introduction to R" which you are requested to have reviewed before
asking questions on Rhelp. Section 11 would be particularly relevant
here.
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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