On 14/04/2010 11:12 AM, Stefan Uhmann wrote:
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)
These look the same to me.
round(fit1$coefficients, 2)
round(fit2$coefficients, 2)
These look different, because poly uses orthogonal polynomials, a
different parametrization. You can see the difference if you ask for
model.matrix(fit1) and model.matrix(fit2). (You can plot these using
matplot(model.matrix(fit1)), etc.)
Duncan Murdoch
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.
