Date: Tue, 1 Mar 2011 18:02:17 +0100
Hello Lorenzo
thank you for the answer. Please have a look the attached figure, I make difference between the two histograms:
blue.Add(red, w)
where w is between -1 and 0. Naively I expect after Add the mean of blue histogram to increase, but it depends on w. I don't understand this results. I verified that there are no bins with negative content before and after Add.
Thanks,
Margar
On Tue, Mar 1, 2011 at 4:14 PM, Lorenzo Moneta <Lorenzo.Moneta_at_cern.ch> wrote:
> Hello Margar,
>
> when you are getting an histogram with negative bins content (for example from the subtraction of two histograms)
> the statistics (mean , s.d., etc..) is computed now in ROOT using the absolute value of the bin content.
> In my opinion, if a bin has negative content, it does not make any sense to compute any statistics using the bin centers.
> You would need to compute it using the original entries from the histogram.
> It is my plan to set artificially a mean/s.d. to zero )or whatever not defined value) in this particular cases to avoid computing a
> totally wrong result and avoiding confusion
>
> Best Regards
>
> Lorenzo
>
>
> On Mar 1, 2011, at 3:54 PM, Margar Simonyan wrote:
>
>> Dear Rene
>>
>> thanks, now I understand the observed differences. In a real example I
>> have another issue, signal+background distribution has empty bins, but
>> background distribution can have non-zero content for the same bins,
>> then the difference has bins with negative content. I tried to re-bin,
>> but the results were depend significantly on grouping. Is there a
>> better way of solving this issue?
>>
>> Can background subtraction from signal+background done with TProfile
>> (Add)? I attach updated version of my script. Certainly TProfile:Add
>> does something different.
>>
>> Best regards,
>> Margar
>>
>> On Tue, Mar 1, 2011 at 12:56 PM, Rene Brun <Rene.Brun_at_cern.ch> wrote:
>>> What you get is perfectly normal.
>>> Following an operation on your histogram (Add, Substract, Rebin, etc) the
>>> statitics for moments (mean, sigma, etc)
>>> are recomputed from the bin contents, assuming the center of the bin.
>>>
>>> Rene Brun
>>>
>>>
>>> On 01/03/2011 12:37, Margar Simonyan wrote:
>>>>
>>>> Hello ROOTTalk
>>>>
>>>> I get strange results after histogram subtraction, the attached script
>>>> written in Python demonstrates the issue. My goal is to subtract
>>>> background from signal+background distribution and get meaningful
>>>> results for mean.
>>>> There are several unexpected (for me) results:
>>>> First, the mean changes after subtracting empty histogram, this is not
>>>> a big issue. Second, after subtracting background I don't get exactly
>>>> the signal value. Third, rebinning before subtracting changes the
>>>> results once more.
>>>> Can somebody explain this? I am using ROOT 5.26/00e complied on SLC5
>>>> with gcc43.
>>>>
>>>> Thanks,
>>>> Margar
>>>> -------------------------------------------------------------------------
>>>> Dr Margar Simonyan, post-doctoral researcher
>>>> Niels Bohr Institute, Copenhagen University
>>>> -------------------------------------------------------------------------
>>>
>>>
>> <histo.py>
>
>
