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

[Petr PIKAL]

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
> 
> > 
> > Thanks in advance,
> > Stefan
> > 
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