Thanks for the clarifications. It seems the confusion resulted from making one assumption more than necessary regarding the behavior of poly().
Best wishes, Firas. On Fri, 2008-03-07 at 18:33 +1000, [EMAIL PROTECTED] wrote: > It does help if you read the help information for poly. > > > ?poly > > x <- 1:3 > > y <- c(1, 4, 9) > > f <- lm(y ~ poly(x, 2, raw = TRUE)) ## note raw = TRUE > > coef(f) > (Intercept) poly(x, 2, raw = TRUE)1 poly(x, 2, raw = TRUE)2 > 0 0 1 > > > > You were assuming a power basis for the polynomial, 1, x, x^2. If you > want to use that you must declare that using raw = TRUE. The default is > to use an orthogonal polynomial basis, and you can expect the > coefficients relative to that to be, well, puzzling. > > Bill Venables > CSIRO Laboratories > PO Box 120, Cleveland, 4163 > AUSTRALIA > Office Phone (email preferred): +61 7 3826 7251 > Fax (if absolutely necessary): +61 7 3826 7304 > Mobile: +61 4 8819 4402 > Home Phone: +61 7 3286 7700 > mailto:[EMAIL PROTECTED] > http://www.cmis.csiro.au/bill.venables/ > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > On Behalf Of Firas Swidan, PhD > Sent: Friday, 7 March 2008 6:16 PM > To: r-help@r-project.org > 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.
