remove all terms with interaction factor in formula
On Sep 13, 2012, at 11:00 AM, Bert Gunter wrote:
~ a*b*d + a*c*d
That seemed pretty clear and obvious, but I started wondering how to tell the machine to do it. Here is another idea:
grep("b:c", attr(terms(~a*b*c*d), "term.labels" ) ,invert=TRUE, value=TRUE)
[1] "a" "b" "c" "d" "a:b" "a:c" "a:d" "b:d" "c:d" "a:b:d" "a:c:d" (Although I realize it's no longer a formula and might need to be reassembled with `paste` and `as.formula`.)
David. > -- Bert > On Thu, Sep 13, 2012 at 10:49 AM, Alexander Shenkin <ashenkin at ufl.edu> wrote: >> Hi Folks, >> >> I'm trying to find a way to remove all terms in a formula that contain a >> particular interaction. >> >> For example, in the formula below, I'd like to remove all terms that >> contain the b:c interaction. >> >>> attributes(terms( ~ a*b*c*d))$term.labels >> [1] "a" "b" "c" "d" "a:b" "a:c" >> [7] "b:c" "a:d" "b:d" "c:d" "a:b:c" "a:b:d" >> [13] "a:c:d" "b:c:d" "a:b:c:d" >> >> My eventual use is to fit models with the reduced formulas. >> >> For example: >>> my_df = data.frame( iv = runif(100), a=runif(100), b=runif(100), >> c=runif(100), d=runif(100)) >>> lm(iv ~ a*b*c*d, data=my_df) >> >> I can remove particular terms with update(), but I don't see a way to >> remove all terms that contain a given combination of factors. >> >> Any help would be greatly appreciated. Thanks! >> >> Allie >> >> ______________________________________________ >> R-help at 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. > > > > -- > > Bert Gunter > Genentech Nonclinical Biostatistics > > Internal Contact Info: > Phone: 467-7374 > Website: > http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm > > ______________________________________________ > R-help at 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. David Winsemius, MD Alameda, CA, USA