Re: Fitting TGraphErrors

From: OKUMURA, Akira <oxon_at_ceres.phys.s.u-tokyo.ac.jp>
Date: Mon, 29 Jun 2009 07:32:08 +0900


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 - 00:32:17 CEST

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