Re: Fitting TGraphErrors

The computation of the chisquare requires of course to call the function in the fitting range.

Rene Brun

OKUMURA, Akira wrote:
> Hello Rene,
>
> you are right. I expected that chi^2 was calculated using only xerror,
> but as you mentioned, it seems that an inverse function does not
> always work in the fitting method. I will modify the code. Thanks a lot.
>
> Regards,
>
> OKUMURA, Akira oxon_at_ceres.phys.s.u-tokyo.ac.jp
> Department of Physics, The University of Tokyo
> 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
> TEL/FAX +81 3-5841-4173/4059
> Skype : okumura.akira
>
> On 2009/06/29, at 2:13, Rene Brun wrote:
>
>> You are generating x values that may be negative !!!
>> then you evaluate an illegal function x**y with eg x=-1.11 and y=1.2
>> Fix your function or the x range.
>>
>> Rene Brun
>>
>> OKUMURA, Akira wrote:
>>> Hello Lorenzo,
>>>
>>> I got a similar problem again when I fit a TGraphErrors with a
>>> function "x**[0]". Would you check the attached macro, please? Most
>>> of the code is the same as the previous one I sent in February.
>>>
>>> Chi^2 often becomes "inf", but not always. Please run the macro
>>> repeatedly if you get a proper result.
>>>
>>> I am using the SVN HEAD on Leopard.
>>>
>>> Regards,
>>>
>>> OKUMURA, Akira oxon_at_ceres.phys.s.u-tokyo.ac.jp
>>> Department of Physics, The University of Tokyo
>>> 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
>>> TEL/FAX +81 3-5841-4173/4059
>>> Skype : okumura.akira
>>>
>>> On 2009/02/25, at 1:36, Lorenzo Moneta wrote:
>>>
>>>> Hi,
>>>>
>>>> using option "F" in your particular case showed a problem. Thank
>>>> you for your example.
>>>> This is now fixed in the trunk version and your macro works fine.
>>>> What stated in the doc is correct, with the linear fitter error in
>>>> X are not taken into account when fitting a TGraphErrors
>>>>
>>>> Best Regards
>>>>
>>>> Lorenzo
>>>> On Feb 24, 2009, at 1:25 PM, OKUMURA, Akira wrote:
>>>>
>>>>> Hello Rene,
>>>>>
>>>>> Did you mention this?
>>>>> =======
>>>>> Note, that the linear fitter doesn't take into account the errors
>>>>> in x. If errors in x are important, go through minuit (use option
>>>>> "F" for polynomial fitting).
>>>>> =======
>>>>>
>>>>> I tried "F" option, but the result becomes chi^2/ndf = 0/0
>>>>>
>>>>> Regards,
>>>>>
>>>>> OKUMURA, Akira oxon_at_ceres.phys.s.u-tokyo.ac.jp
>>>>> Department of Physics, The University of Tokyo
>>>>> 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
>>>>> TEL/FAX +81 3-5841-4173/4059
>>>>> Skype : okumura.akira
>>>>>
>>>>> On 2009/02/24, at 21:03, Rene Brun wrote:
>>>>>
>>>>>> see comments in the doc of TGraph::Fit
>>>>>> http://root.cern.ch/root/html/TGraph.html#TGraph:Fit
>>>>>>
>>>>>> Rene Brun
>>>>>>
>>>>>> OKUMURA, Akira wrote:
>>>>>>> Hello ROOTers,
>>>>>>>
>>>>>>> I attache an example of TGraphErrors::Fit. The left plot returns
>>>>>>> reasonable chi^2/ndf, but the right side doesn't. The result
>>>>>>> depends on the variable "kScale" in the macro.
>>>>>>>
>>>>>>> The fitting results from the left and the right plots should be
>>>>>>> the same value mathematically.
>>>>>>>
>>>>>>> Am I doing something incorrect?
>>>>>>>
>>>>>>> 5.22.00 + Mac OS X 10.5.6
>>>>>>>
>>>>>>> Regards,
>>>>>>>
>>>>>>> OKUMURA, Akira oxon_at_ceres.phys.s.u-tokyo.ac.jp
>>>>>>> Department of Physics, The University of Tokyo
>>>>>>> 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
>>>>>>> TEL/FAX +81 3-5841-4173/4059
>>>>>>> Skype : okumura.akira
>>>>>>
>>>>>
>>>>
>>
>
Received on Mon Jun 29 2009 - 07:25:48 CEST