Re: [R] Odp: fitting a quadratic function - poly?
Hi Bert Gunter gunter.ber...@gene.com napsal dne 14.04.2010 18:54:37: Petr Pikal wrote: ... I mean that you can use fit- lm(y~x+I(x^2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 but you can not use fit- lm(y~poly(x,2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 (to get the fits for any x vector) -- But you **can** use ypred - predict(fit,data.frame(x = x)) -- in **both** cases. Which is, I think, how it should be done. I completely agree with you. However sometimes it is necessary to put an equation and estimated coefficients to a report - for that you can not use coefficients from poly() estimate directly. (Please do not beat me, I know that polynomial model is ***rarely*** physically viable and personally I never use it until it it has not a physical sense - like free fall) Regards Petr Cheers, Bert Bert Gunter Genentech Nonclinical Statistics answer is, yes you can. See ?SafePrediction for details. -- Bert Regards Petr Thanks in advance, Stefan __ 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. __ 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] Odp: fitting a quadratic function - poly?
Hi r-help-boun...@r-project.org napsal dne 14.04.2010 17:12:51: Hi List, I can not get my head around the following problem. I want to fit a quadratic function to some data and stumbled across poly(). What exactly does it, i.e. why are there different results for fit1 and fit2? x = seq(-10, 10) y = x^2 fit1 = lm(y ~ x + I(x^2)) fit2 = lm(y ~ poly(x, 2)) plot(x,y) lines(x, fit1$fitted.values, col = 2) lines(x, fit2$fitted.values, col = 3) round(fitted(fit1)-fitted(fit2),5) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 results are same. round(fit1$coefficients, 2) round(fit2$coefficients, 2) Coefficients are different as you fit different values. See ?poly poly(-10:10,2) I believe that others give you better explanation. So you can not use coefficients evaluated by lm(.~poly(...)) directly. Regards Petr Thanks in advance, Stefan __ 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.
Re: [R] Odp: fitting a quadratic function - poly?
Below. -- Bert Bert Gunter Genentech Nonclinical Statistics Coefficients are different as you fit different values. See ?poly poly(-10:10,2) I believe that others give you better explanation. So you can not use coefficients evaluated by lm(.~poly(...)) directly. -- Well, it depends what you mean by use...directly. But I think the answer is, yes you can. See ?SafePrediction for details. -- Bert Regards Petr Thanks in advance, Stefan __ 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. __ 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.
Re: [R] Odp: fitting a quadratic function - poly?
Hi Bert Gunter gunter.ber...@gene.com napsal dne 14.04.2010 18:01:52: Below. -- Bert Bert Gunter Genentech Nonclinical Statistics Coefficients are different as you fit different values. See ?poly poly(-10:10,2) I believe that others give you better explanation. So you can not use coefficients evaluated by lm(.~poly(...)) directly. -- Well, it depends what you mean by use...directly. But I think the I mean that you can use fit- lm(y~x+I(x^2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 but you can not use fit- lm(y~poly(x,2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 to compute y. Regards Petr answer is, yes you can. See ?SafePrediction for details. -- Bert Regards Petr Thanks in advance, Stefan __ 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. __ 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.
Re: [R] Odp: fitting a quadratic function - poly?
Petr Pikal wrote: ... I mean that you can use fit- lm(y~x+I(x^2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 but you can not use fit- lm(y~poly(x,2)) coef(fit)[1] + coef(fit)[2]*x + coef(fit)[3]*x^2 (to get the fits for any x vector) -- But you **can** use ypred - predict(fit,data.frame(x = x)) -- in **both** cases. Which is, I think, how it should be done. Cheers, Bert Bert Gunter Genentech Nonclinical Statistics answer is, yes you can. See ?SafePrediction for details. -- Bert Regards Petr Thanks in advance, Stefan __ 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. __ 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.
