Emily,
It looks like your histogram bin errors are not defined
correctly.
By default (if TH1::Sumw2 was not called before filling),
the bin error will be the sqrt(bin content).
In case you have filled your histograms with weights
or have made an operation (add,multiply,divide,scale)
and you did not call TH1::Sumw2, your errors are
meaningless. The fit parameters may be OK, but
certainly not the chisquare and the errors on the
parameters.
So, check this point first. If you do not find a solution,
send me a Root file containing the histogram you want
to fit together with a small script containing
your fitting function and the calls to TH1::Fit
Rene Brun
On Fri, 19 Apr 2002, Emily Nurse wrote:
> Hi Rene,
>
> I am fitting a user defined function (just the bin content from a
> histogramme as a function of x-axis) to data points. I want to do it
> without changing the normalisation scale how-ever when I use the option
> "w" my chisquared value is too large. I have checked by getting the
> integral of my function before and after fitting without the "w" option so
> as to get the normalisation constant. I then weight my function by the
> ratio of these integrals and then make a fit with the "w" option. My
> chisquared value is nearly a factor 10 higher when really it should be the
> same. Can you explain?
> Cheers,
> Emily.
>
>
> On Fri, 15 Mar 2002, Rene Brun wrote:
>
> > Hi Emily,
> >
> >
> >
> > On Thu, 14 Mar 2002, Emily Nurse wrote:
> >
> > > Hello,
> > > I have a couple of questions involving the Fit function.
> > > Firstly when I use it it normalises my user defined function for me to
> > > give the loest possible chisquared value when fitted to my data points. I
> > > was wandering if I could switch this action off and just get a straight
> > > forward chisquared value as I will be fitting different ranges seperately
> > > that need to be normalised by the same amount even though this may not
> > > give the lowest chi-squared value.
> >
> > If you use the fit option "w" (all weights equal to 1), you should obtain
> > what you want.
> >
> > >
> > Secondly I am getting an error message when I run my macro which includes
> > > the function:
> > >
> > > Double_t norm ( Double_t *x, Double_t *par)
> > > {
> > > Double_t xx=x[0];
> > > Int_t bin = w_pte1->GetXaxis()->FindBin(xx);
> > > return par[0]*(w_pte1->GetBinContent(bin));
> > > }
> > >
> > > (where w_pte1 is a TH1D object). Then at the root prompt I type:
> >
> > But where do you define it ?
> > If the code above is in a macro, declare your TH1D object as global.
> >
> > Rene Brun
> >
> > >
> > > root [3] TF1 *f1 = new TF1("norm",norm,0.,50.,1)
> > > root [5] f1->SetParameter(0,1)
> > > root [6] f1->Draw()
> > > <TCanvas::MakeDefCanvas>: created default TCanvas with name c1
> > > Error: non class,struct,union object $w_pte1 used with . or ->Error: non
> > > class,struct,union object $w_pte1 used with . or ->Error: non
> > > class,struct,union object $w_pte1 used with . or ->root [7]
> > >
> > > Things do seem to work but I was wandering what I was doing wrong,
> > > thanks for your help,
> > > Emily.
> > >
> >
>
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