I think you need poly(X, 2, raw = TRUE) to interpret the coefficients
in the manner described below.
poly uses orthogonal polynomials by default:
poly package:stats R Documentation
Compute Orthogonal Polynomials
Description:
Returns or evaluates orthogonal polynomials of degree 1 to
'degree' over the specified set of points 'x'. These are all
orthogonal to the constant polynomial of degree 0. Alternatively,
evaluate raw polynomials.
David Scott
On 21/01/2011 3:50 a.m., Joshua Wiley wrote:
Hi Barth,
Here is an option fitting a linear model toa second order polynomial
and extracting the coefficients. The Intercept corresponds to "c" in
your email, then poly(...)1 to "b" and poly(...)2 to "a".
################
dat<- read.table(textConnection("
Y X
0.159529 0
0.5 0.773019
1 1"), header = TRUE)
closeAllConnections()
coef(lm(Y ~ poly(X, 2), data = dat))
#################
For details see:
?poly
?lm
?coef
Hope this helps,
Josh
On Thu, Jan 20, 2011 at 6:42 AM, Barth B. Riley<bbri...@chestnut.org> wrote:
Hello
I would like to obtain the coefficients for a quadratic function (ax^2 + bx +
c) given three sets of points on the quadratic curve. For instance:
Y X
0.159529 0
0.5 0.773019
1 1
Is there a function in R to obtain the a, b and c ceofficients?
Thanks
Barth
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