[R] Obtaining a quadratic function igven three points on a curve
d.scott at auckland.ac.nz
Thu Jan 20 16:17:52 CET 2011
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
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.
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)
> coef(lm(Y ~ poly(X, 2), data = dat))
> For details see:
> Hope this helps,
> On Thu, Jan 20, 2011 at 6:42 AM, Barth B. Riley<bbriley at chestnut.org> wrote:
>> 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?
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David Scott Department of Statistics
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