Re: histogram subtraction and mean

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
>>>  -------------------------------------------------------------------------
>> 
>>