Really strange Fit behaviour

ROOT
1

Hello,

I see a really strange fit behaviour, which is illustrated by the attached macro, which fits a polarization signature with a cosine.

Please play with the number of bins (NBins) of the histogram:
At NBins=100, the correct modulation is retrieved (0.20)
At NBins=1000, the modulation is somewhat smaller (0.16) but not even within errors close to the input
AT NBins=10000, the fitted cosine is almost flat, but visually the correct modulation is clearly visible (see attached image).

Any idea what might be wrong? The chi-square of the fit looks reasonable, but the result is definitely not…

Strangely, if one deactivates the normalization (UseNormalization=false) in the macro, the fit becomes acceptable again.

Any ideas what might be wrong?

Andreas

OS: SuSE 9.2
CC: icc
ROOT: 4.04/02 (not cvs)
FitProb.C (2.41 KB)

2

Andreas,

Your normalization is wrong. I assume that instead of
Hist->SetBinContent(b, Hist->GetBinContent(b)/Mean-1);
you meant
Hist->SetBinContent(b, Hist->GetBinContent(b)/(Mean-1));

Rene

3

Rene,

The normalization is only for illustration - and it is as intended - the values should be above and below zero.

The problem is - if there are values above and below zero - as in the example - then the fit misbehaves, i.e. it works perfectly ok if all values are positive or all values are negative.

Andreas

4

Andreas,

You should define correct errors or force equal weights for all bins,
eg replace
Hist->Fit(Mod, “R”);
by
Hist->Fit(Mod, “Rw”);

Rene

5

Now it works!

Thanks! :smiley: