Fitting without errors (works)
I made a simple linear fit in Gnuplot 5.0 using the command:
f(x)=a*x+b
fit f(x) 'file.dat' using 1:2 via a,b
I get the output:
degrees of freedom (FIT_NDF) : 6
rms of residuals (FIT_STDFIT) = sqrt(WSSR/ndf) : 0.00794747
variance of residuals (reduced chisquare) = WSSR/ndf : 6.31623e-05
Final set of parameters Asymptotic Standard Error
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p1 = -0.00964423 +/- 0.0004976 (5.159%)
p2 = 1.07794 +/- 0.01908 (1.77%)
The result is this:
Fitting with errors
Then I added very tiny error bars just to see how they influence the fitting results (expecting the difference from the previous case to be very small), but using the command
f(x)=a*x+b
fit f(x) 'file.dat' using 1:2:3 yerrors via a,b
I get a completely wrong fit:
The output is:
degrees of freedom (FIT_NDF) : 6
rms of residuals (FIT_STDFIT) = sqrt(WSSR/ndf) : 750.565
variance of residuals (reduced chisquare) = WSSR/ndf : 563348
p-value of the Chisq distribution (FIT_P) : 0
Final set of parameters Asymptotic Standard Error
======================= ==========================
p1 = -0.0115247 +/- 0.0003419 (2.967%)
p2 = 1.15636 +/- 0.01483 (1.282%)
Furthermore, if I set the errors to be much larger, the output remains the one I had for tiny errors. Do anyone have suggestions? What did I do wrong?
Data
y x dy
0.64345112296614271 45.082768716145587 6.6513808914832773E-004
0.71703932263695935 38.322543680055119 1.8140129703996476E-004
0.62214826712778870 46.283953074076770 1.2093419803380392E-004
0.70999997854232788 39.152893923419398 3.9303614359375108E-004
0.75723404482236245 33.204658354605364 6.6513808915369822E-004
0.69366599317566635 39.410047372618159 5.8653086043387384E-003
0.75948892906677234 33.491967428263528 6.6513808915369822E-004
0.79365751671683227 28.533494222921814 1.2758557891916475E-002
where for the first plot I just used the first two columns and for the second one I used the third for the error in y.
