A good article to read before using ratios is: Kronmal, RA. Spurious Correlation and the Fallacy of the Ratio Standard Revisited. Journal of the Royal Statistical Society. Series A, Vol 156, No. 3 (1993), 379-392.
It shows some of the traps that misusing ratios can lead to along with strategies for avoiding those traps. Hope this helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Jeroen Ooms > Sent: Thursday, December 18, 2008 3:03 AM > To: r-help@r-project.org > Subject: [R] a ratio-variable predictor > > > Maybe this is more of a statistical question than an R question, but I > am > going to ask it anyway :) Cortisol and Testosteron are known to > interact in > the body, and some literature suggest that especially the ratio between > the > two is a good predictor. So I want to add the ratio predictor > (y~cort/test) > to a glm, and later a multilevel model, however I have no experience > with > ratio variables. > > Intuitively, I would think that a ratio is very similar to an > interaction > effect, only with an inverse scale for the second term (cort* 1/test). > However, I found that even more than with normal interaction effects, > the > scaling of the variables becomes important. Furthermore there is the > obvious > problem (which is not too big of a problem in my case), of values that > are > close to zero on the second term. > > So what is an appropriate way to incorporate a ratio-predictor? Any > tips on > R procedure/package that I could use, or any other experiences with > ratio-predictors are welcome. > -- > View this message in context: http://www.nabble.com/a-ratio-variable- > predictor-tp21069805p21069805.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help@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. ______________________________________________ R-help@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.
