Try: glm( y ~ x + I( (x-20)*(x>20) ), ...
This will give you a piecewise linear function with a change point at x=20 and the 2 lines meeting at x=20. The intercept and slope on x will represent the line for x <= 20 and the slope on the I(...) term will represent the difference in slope between the previous line and the new slope for x>=20 (so the test on this term tests the null that a single line fits as well as the piecewise linear). Hope this helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare [EMAIL PROTECTED] (801) 408-8111 > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of zhijie zhang > Sent: Tuesday, January 01, 2008 8:25 AM > To: [EMAIL PROTECTED] > Subject: [R] Specify a correct formula in R for Piecewise > Linear Functions? > > Dear all, > I have two variables, y and x. It seems that the > relationship between them is Piecewise Linear Functions. The > cutpoint is 20. That is, when x<20, there is a linear > relationship between y and x; while x>=20, there is another > different linear relationship between them. > How can i specify their relationships in R correctly? > # glm(y~I(x<20)+I(x>=20),family = binomial, data = point) > something like this? > Thanks a lot. > > -- > With Kind Regards, > > oooO::::::::: > (..)::::::::: > :\.(:::Oooo:: > ::\_)::(..):: > :::::::)./::: > ::::::(_/:::: > ::::::::::::: > [************************************************************* > **********] > Zhi Jie,Zhang ,PHD > Tel:+86-21-54237149 > Dept. of Epidemiology,School of Public Health,Fudan > University Address:No. 138 Yi Xue Yuan Road,Shanghai,China > Postcode:200032 > Email:[EMAIL PROTECTED] > Website: www.statABC.com > [************************************************************* > **********] > oooO::::::::: > (..)::::::::: > :\.(:::Oooo:: > ::\_)::(..):: > :::::::)./::: > ::::::(_/:::: > ::::::::::::: > > [[alternative HTML version deleted]] > > ______________________________________________ > 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.
