[R] lm coefficients

[Sundar Dorai-Raj]

Look at ?poly. It's doing something your not expecting.

What you want is

lm(T ~ X + I(X^2), data = d)

Regards,
Sundar


Timur Elzhov wrote:

> Dear R experts,
> 
> Excuse me if my question will be stupid...
> I'd like to fit data with x^2 polynomial:
> 
> d <- read.table(file = "Oleg.dat", head = TRUE)
> d
>   X         T
>   3720.00   4.113
>   3715.00   4.123
>   3710.00   4.132
>   ...
> 
> out <- lm(T ~ poly(X, 4), data = d)
> out
>   Call:
>   lm(formula = T ~ poly(X, 2), data = d)
>   
>   Coefficients:
>   (Intercept)  poly(X, 2)1  poly(X, 2)2  
>         9.803     -108.075       51.007  
> 
> So, d$T best fitted with function
>   9.803 -108.075 * X + 51.007 * X^2,
> yes?
> 
> T1 <- 9.803 -108.075 * d$X + 51.007 * d$X^2
> T1
>   705453240
>   703557595
>   701664500
>   699773956
>   ...
> 
> So, T1 obviosly gets non-sensible values.. :( Why?
> Thanks a lot!
> 
> --
> WBR,
> Timur.
> 
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