Re: histogram subtraction and mean

When you use TH1::Fill, ROOT can compute the moments precisely because it has the original input values.
As soon as you make an operation (add, ssubtract, zoom, etc), the only thing that we can do is to start from
the bin contents only with the approximation that all values in the bin are at the center of the bin.

Rene

On 01/03/2011 15:50, Stefan Piperov wrote:
>
> Well, probably the more interesting question is why before operations
> the moments are not calculated using the bin contents?
> That's a default which has always puzlled me.
> Stefan.
>
> On Tue, 1 Mar 2011, Rene Brun 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
>>>
>>> -------------------------------------------------------------------------
>>
>>
>>
>
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Received on Tue Mar 01 2011 - 15:54:36 CET