If I fit the data

1 2.5

3 2.1

6 8

10 17.01

using

f(x) = a*x+b

I get

--------------------------------------------------

After 1 iterations the fit converged.

final sum of squares of residuals : 9.65289

rel. change during last iteration : -1.84023e-016

degrees of freedom (FIT_NDF) : 2

rms of residuals (FIT_STDFIT) = sqrt(WSSR/ndf) : 2.19692

variance of residuals (reduced chisquare) = WSSR/ndf : 4.82645

Final set of parameters Asymptotic Standard Error

======================= ==========================

a = 1.71413 +/- 0.3239 (18.9%)

b = -1.16815 +/- 1.957 (167.5%)

correlation matrix of the fit parameters:

a b

a 1.000

b -0.828 1.000

--------------------------------------------------

The fit seems to work, but if I change the input data, the correlation matrix does not change!? I always get -0.828.

Or do I misunderstand something and this is correct?

Thank you for answers,

Sammylight