*example*( pmin ) is useful. I will use it from now on. Thanks a lot. On Jan 3, 2008 2:50 AM, Charles C. Berry <[EMAIL PROTECTED]> wrote:
> On Thu, 3 Jan 2008, zhijie zhang wrote: > > > Some developments with confusions. I tried the spline method and dummy > > variable approach to do it. But their results are very different. See > > following. > > > > [volumes of output and gratuitous SAS code deleted] > > > > > Q1: Why are these two methods so different for the results, e.g. the > > coefficients? > > > For the same reason that Thomas replied to my email suggesting a different > approach than the one I showed you. viz. The spline basis differs from the > basis vectors he constructed. > > > > > Q2: The spline method is useful for piecewise linear functions, e.g. > > bs(distance_trans,degree=1,knots=c(13,25)), > > but how should i do if i want to fit a linear function for the case the > > distance_trans<13,and quadratic curve when distance_trans>=13? > > "bs(distance_trans,degree=c(1,2),knots=13)" cannot works. And even for > more > > than three parts. <13,13~25, >25. > > > > Whew! My response would be "don't go there". Fit a richer basis than you > need and use penalization to damp out unneeded variation in the fit. Or > use GAMs. > > But if you feel you must, you can construct things like > > bs( pmax( 13, pmin( 25 , x ) ) > > > > Q3:"fit <- glm( y ~ pmax(x,20)+pmin(x,20), family=binomial)" is good. > But if > > i divide x into three or more parts, how should i specify it in this > way? > > > > As above. > > > Hope somone can help.Thanks a lot. > > > > You can help youself a lot by taking a few minutes to learn to do in R > what you did in SAS. Reading the help pages AND running the examples is > often illuminating. For example, > > example( pmin ) > > should give you some helpful hints. > > HTH, > > Chuck > > > > > > > > On Jan 2, 2008 11:58 PM, Thomas Lumley <[EMAIL PROTECTED]> wrote: > > > >> On Tue, 1 Jan 2008, Charles C. Berry wrote: > >>> On Tue, 1 Jan 2008, zhijie zhang wrote: > >>> > >>>> 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? > >>> > >>> Try this: > >>> > >>>> library(splines) > >>>> fit <- glm( y ~ bs( x, deg=1, knots=20 ), family=binomial) > >>> > >> > >> In the linear case I would actually argue that there is a benefit from > >> constructing the spline basis by hand, so that you know what the > >> coefficients mean. (For quadratic and higher order splines I agree that > >> pre-existing code for the B-spline basis makes a lot more sense). > >> > >> For example, in > >> fit <- glm( y ~ pmax(x,20)+pmin(x,20), family=binomial) > >> the coefficients are the slope when is < 20 and the slope when x>20. > >> > >> -thomas > >> > >> > >> Thomas Lumley Assoc. Professor, Biostatistics > >> [EMAIL PROTECTED] University of Washington, Seattle > >> > > > > > > > > -- > > 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 <http://www.statabc.com/> > > > [***********************************************************************] > > oooO::::::::: > > (..)::::::::: > > :\.(:::Oooo:: > > ::\_)::(..):: > > :::::::)./::: > > ::::::(_/:::: > > ::::::::::::: > > > > Charles C. Berry (858) 534-2098 > Dept of Family/Preventive > Medicine > E mailto:[EMAIL PROTECTED] UC San Diego > http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901 > > > -- 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.
