poly() computes by default orthogonal polynomials; check the online help file for poly() for more info. Probably you want to use the 'raw' argument in this example, i.e.,
x <- 1:3 y <- c(1, 4, 9) lm(y ~ poly(x, 2, raw = TRUE)) I hope this helps. Best, Dimitris ---- Dimitris Rizopoulos Biostatistical Centre School of Public Health Catholic University of Leuven Address: Kapucijnenvoer 35, Leuven, Belgium Tel: +32/(0)16/336899 Fax: +32/(0)16/337015 Web: http://med.kuleuven.be/biostat/ http://www.student.kuleuven.be/~m0390867/dimitris.htm ----- Original Message ----- From: "Firas Swidan, PhD" <[EMAIL PROTECTED]> To: <r-help@r-project.org> Sent: Friday, March 07, 2008 9:16 AM Subject: [R] Puzzling coefficients for linear fitting to polynom > Hi, > > I can not comprehend the linear fitting results of polynoms. For > example, given the following data (representing y = x^2): > >> x <- 1:3 >> y <- c(1, 4, 9) > > performing a linear fit > >> f <- lm(y ~ poly(x, 2)) > > gives weird coefficients: > >> coefficients(f) > (Intercept) poly(x, 2)1 poly(x, 2)2 > 4.6666667 5.6568542 0.8164966 > > However the fitted() result makes sense: > >> fitted(f) > 1 2 3 > 1 4 9 > > This is very confusing. How should one understand the result of > coefficients()? > > Thanks for any tips, > Firas. > > -- > Firas Swidan, PhD > Founder and CEO > Olymons: Blessing Machines with Vision (TM) > http://www.olymons.com > P.O.Box 8125 > Nazareth 16480 > Israel > Cell: +.972.(0)54.733.1788 > > ______________________________________________ > 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. > Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm ______________________________________________ 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.