fitting 1D data to y=x polynomial (cftool)

In an experiment vector y is a measurement of vector x. I want to fit polynomial y=x to this data and find R^2. When I use polyfit(x,y,1), MATLAB gives me y=ax+b but I am interested in knowing the R^2 when the data is fitted to y=x. Could someone please help me on that?

I found cftool which can solve my problem. Could someone tell me how I can retrieve R^2 from the analysis of cftool when the data is fitted to a polynomial?

Thanks.

[Jesús Lucio]

Hello. I think you only have to evaluate the fitted polynomial at the same abscissas and compare with the original ordinates:

poly = polyfit(x, y, 1);

yfit = polyval(poly, x);

cc = corrcoef(y, yfit);

R2 = cc(1, 2).^2;

Answer 1

[Sean de Wolski]

Matt_Tearle's_Answer

[AP]

I know how to fit y=ax+b but I want to fit the curve y=x to my data. Do you know how I can use cftool at MATLAB command prompt?

[Sean de Wolski]

No reason for cftool. Did you look at Matt's answer?

[AP]

I did. But it does not fit to y=x. It fits it to y=mx+b. I solved my problem by using fit command.

[Sean de Wolski]

All you need to do is change the vandermonde matrix, in his example 'F'

[Sean de Wolski]

For zeroth order fit; i.e. what you want, it will just be x.^0, aka ones(size(x)) aka the mean.