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.