root version 3.05/07
I am using the root Poisson function:
i do:
poiss2=new TF1("poiss2","TMath::Poisson(x,[1])",0,99);
float x0=40;
poiss2->SetParameter(1,x0);
poiss2.Integral(x0,99)
>> this returns (Double_t)4.89489910522337757e-01
>> ok so i expect here in the gaussian limit there to be 50% of the
>>distribution >X0, this answer is close enough i guess?
now i do:
float x0=0.3;
poiss2->SetParameter(1,x0);
poiss2.Integral(x0,99)
>>this returns (Double_t)3.87128082086576730e-01
>> meaning that 39% of the distribution lies above X0.
Is this true or am i using the function in a region where it becomes
non-valid?
ie. is there really not 50% of the distribution either side of the
parameter X0 in a Poisson distribution when X0 is small? (sorry, this is a
maths question but i could'nt find a comprehensive answer in my stats
book..) If it is a problem with the root approximation than can you give
me an idea of the range for which i can use it?
cheers,
Dan.
--
CMS-Bristol
http://cern.ch/dan.holmes
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