Re: histogram subtraction and mean

From: Stefan Piperov <piperov_at_fnal.gov>
Date: Tue, 1 Mar 2011 18:43:44 +0200

Hi Rene,

I don't want to argue here - after all you are the author of ROOT, not me - but it would be interesting to hear to opinions of both user community, and professional statistiticians on this subject.

I clearly see the implications that such a change will have on all existing ROOT analysis codes, so I'm not proposing this change lightly.

With Best Regards,
Stefan.

On Tue, 1 Mar 2011, Rene Brun wrote:

> No, I must disagree with you. ROOT computes the best possible value if the
> necessary information is available.
> I am sure that we will get zillions of complaints if we were following your
> suggestion ::)
>
> Rene
>
>
> On 01/03/2011 15:59, Stefan Piperov wrote:
>>
>> I understand that mechanism, and the fact that computing moments from
>> un-binned data is more precise, but it's also misleading. If I'm filling a
>> histogram, then I want to know the charachteristics (e.g. moments) of the
>> binned data, not the originals. I can always calculate the moments of the
>> unbinned data if I wished.
>>
>> To me, at least, the default should be moments to be calculated from
>> binned data. The other behaviour should be optional.
>> But that might be quite a change on ROOT...
>>
>> Stefan.
>>
>>
>>
>> On Tue, 1 Mar 2011, Rene Brun wrote:
>>
>> > 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
>> > > > >
>> > > -------------------------------------------------------------------------
>> > > > > >
>> > > *---------------------------------------------------------------------*
>> > >
>> > > Stefan Piperov Mail: FNAL P.O.Box 500, MS 205, Batavia,
>> > > IL-60510
>> > > Phone: (630) 840-5176 E-Mail:
>> > > piperov_at_fnal.gov
>> > > *---------------------------------------------------------------------*
>> > >
>> > > "Give a skeptic an inch... and he'll measure it."
>> > >
>> >
>> >
>>
>> *---------------------------------------------------------------------*
>> Stefan Piperov Mail: FNAL P.O.Box 500, MS 205, Batavia, IL-60510
>> Phone: (630) 840-5176 E-Mail: piperov_at_fnal.gov
>> *---------------------------------------------------------------------*
>> "Give a skeptic an inch... and he'll measure it."
>>
>
>
>

*---------------------------------------------------------------------*
  Stefan Piperov      Mail: FNAL P.O.Box 500, MS 205, Batavia, IL-60510
  Phone: (630) 840-5176                        E-Mail: piperov_at_fnal.gov
*---------------------------------------------------------------------*

"Give a skeptic an inch... and he'll measure it." Received on Tue Mar 01 2011 - 17:43:52 CET