Re: Fit options in ROOT

Hi,

  there are currently no tutorials on doing this. It is something we should add it.
I have attached an example using the option "U" when fitting

  Best Regards

  Lorenzo

On Nov 6, 2009, at 12:58 PM, Paolo Risso wrote:

> Hi all,
>
> Sorry to add myself in the question, but
> Is there any guided example about it ?
>
> Thanks
>
> Paolo
>
> Il giorno 06/nov/09, alle ore 12:12, Lorenzo Moneta ha scritto:
>
>> Hi,
>>
>> you cannot change with a simple option the errors in the chi2
>> calculations. You would need to write yourself the chi2 function
>> and pass to the fit,
>> using for example the option "U".
>> You can however use the option "L" to perform a binned likelihood
>> fit. In this case you will have the correct treatment of the bins
>> statistics in particular if you have empty bins
>>
>> Best Regards
>>
>> Lorenzo
>> On Nov 5, 2009, at 5:20 PM, Jordi Nadal Serrano wrote:
>>
>>> Hello guys,
>>>
>>> I am trying to fit a function to a histogram. Like this:
>>>
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>>
>>> Double_t myfunction (Double_t *x, Double_t *par){
>>> int bin = data_8175->GetXaxis()->FindBin(x[0]);
>>> Double_t f = 0;
>>> f= par[0] * ttbar->GetBinContent(bin) + par[1] * h_w_jets-
>>> >GetBinContent(bin) + par[2] * h_others->GetBinContent(bin);
>>> return f;
>>>
>>> }
>>>
>>>
>>> void myfunc(){
>>>
>>> TF1 *f1 = new TF1("myfunc",myfunction,minBin_fit,maxBin_fit,3);
>>>
>>> f1->SetParLimits(0,0,2);
>>> f1->SetParLimits(1,0,2);
>>> f1->FixParameter(2,1);
>>> f1->Draw();
>>> }
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>>
>>> Once my histograms are filled. I do the fit this way:
>>>
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>> myfunc();
>>> TF1 *f1= (TF1*)gROOT->GetFunction("myfunc");
>>> data_8175->Fit("myfunc","ER0");
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>>
>>>
>>> I wonder what are the errors that are entered in the chi square. I
>>> believe that the errors are the sqrt( number of entries in a bin )
>>>
>>> Can I change these errors to be the theoretical ones, i.e, the
>>> sqrt of the value of myfunction per bin .
>>>
>>>
>>> Many thanks in advance,
>>>
>>> Jordi
>>>
>>
>
> Paolo Risso
> Universita' di Genova
> INFN sezione di Genova
> Via Dodecaneso 33
> 16146 - Genoa - Italy
> tel +390103536452 (office)
> paolo.risso_at_ge.infn.it
>
>
>