# Fitting a Linear Function

Hi All,
I’m a ROOT newbie and trying to fit a simple linear function but the values of parameters and chi^2/ndf are not coming nice. What am I missing?

I’ve uploaded the cpp file, data file and graph png below.

Thanks.

test.cpp (328 Bytes)
test2.txt (25 Bytes)

ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

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Everything is perfectly fine.

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Thanks for replying @Wile_E_Coyote I understand why chi^2 is zero but isn’t the ratio chi^2/ndf must be equals to 1 for a good fit. In other words, what’s the significance of this ratio, does it has to be 1 every time?

Also If I slightly change the initial parameters : f1->SetParameters(3,3)

the values of p0, p1 and chi^2 comes out to be drastically different. Can you please explain it?

Thanks.

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For how to “interpret” chi^2 (and chi^2 / NDF) you need to look into some statistics book.

I don’t get any problem if I set initial parameters (3,3). For me, it returns Chi2=2.3e-22 (“almost” 0), p0=5±2.8e-12 and p1=3e-12±9.2e-12 (“almost” 0).

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Ok! Here I knew the slope and intercept beforehand but in cases where I don’t, how would I know that the parameter values are “almost” correct? Should I have to completely rely on chi^2/NDF ratio?

You always get “parameter” ± “parameter_error” (i.e. you never get any “true” / “real” / “undisputed” / “absolute” values).

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Ok! Thanks @Wile_E_Coyote for your help!

In ROOT, before you try to fit your graph or histogram, you MUST set “reasonable” initial values for ALL parameters of your function (except for some “built-in” formulas, like “gaus”, for which the standard fit procedure can automatically “guess” them), otherwise the fitting procedure may easily misbehave.

Search for “Setting initial conditions” in the TGraph::Fit and TH1::Fit methods descriptions.

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