Polyfit - Forcing a non-zero intercept

Hi,

Is there any way to specify the y-intercept value of polyfit? I have had a look around, and when this question is asked most people are trying to fit it to a 0 y-intercept. In my case, I need to fit least-square lines to several different groups of points whose y-intercepts are known and are non-zero.

Any help would be great,

FP

Answer 1

[Star Strider]

I doubt that's possible with polyfit. If you are only using a first order regression model and you have access to one of the nonlinear curve fitting routines ( nlinfit in the Statistics Toolbox or lsqcurvefit in the Optimization Toolbox), the easiest way to do this is simply to use them with an objective function similar to:

Intercept = 5;
RegModel = @(B,X) B(1).*X + Intercept;

then:

beta0 = rand;
[Beta,R,J,CovB,MSE] = nlinfit(X, Y, RegModel, beta0);

You can use whatever sort of regression model you want, of course. In this one I used a simple single-parameter model.

You can specify whatever you want in a model function, and can fit as many parameters as you want (that are mathematically appropriate).

[F P]

Cheers

Answer 2

[Matt J]

So you just have a slope to estimate? It should be just

    slope=xData\(yData-yIntercept);