# [R] Odp: fitting a quadratic function - poly?

Petr PIKAL petr.pikal at precheza.cz
Wed Apr 14 18:16:53 CEST 2010

Hi

Bert Gunter <gunter.berton at 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
>
> >
> > Stefan
> >
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