Re: Map the chi2

Hi Manuel,

What you require should be quite easy using: gMinuit->Eval(Int_t npar, Double_t *grad, Double_t &fval, Double_t *par, Int_t flag);

Create a TH3 (or TGraph2D), make your nested 3 loops on each parameter in the limits you want.
In the loop call gMinuit->Eval and store the returned value fval in your TH3 or TGraph2D.

Rene Brun

Manuel Mussini wrote:
> Hi Lorenzo,
> contours scans only "near" the found minimum so I think it isn't the right
> choice for searching for another eventual "far" minimum!
> I've never tried SCAN! I'll do it as my next step!
> Is it possible to scan "by hand"? I mean ... can I fix all my 3 parameters and
> evaluate the chi2? With TMinuit::FixParameter(Int_t) I think I can get the first
> part but ... how to update the chi2?
> The best result would be to obtain a 3D histo showing the chi2 surface (for each
> parameters couple)!
>
>
> Bye…
>
>
>
> ________________________________________
> From: Lorenzo Moneta [mailto:Lorenzo.Moneta_at_cern.ch]
> Sent: Monday, March 10, 2008 23:14
> To: Manuel Mussini
> Cc: roottalk_at_lxbuild091.cern.ch
> Subject: Re: [ROOT] Map the chi2
>
> Hi Manuel,
>
> you can either scan a single parameter around the minimum, (command SCAN in
> Minuit) or build contours for two parameters, like in fitcont.C.
> I think contours will give you the best way for maping the chi2 around the
> minimum.
>
> Cheers
>
> Lorenzo
> On Mar 10, 2008, at 7:20 PM, Manuel Mussini wrote:
>
>
> Hi,
> I have a TH1D and a 3 parameters user defined TF1 to fit it.
> Which is the best way to map the chi2 around the convergence point?
> I thoughed to map the 3 parameters in pairs (fix one of them and scan the other
> two) but it not so clear (to me!) how to implement the mapping!
> I’ve tried the fitcont.C tutorial but this is not what I want!
>
> Thanks!!!
>
> Bye…
>
>
>
Received on Tue Mar 11 2008 - 08:33:42 CET