Hi Saleem, When you fit a TH1F histogram (for example named "hist") that has N events with a Gaussian: hist->Fit("gaus"); the Gaussian function is normalized such that it will give you the number of events inside the histogram. Doing some math, if G is a Gaussian with some constant term (c), mean (m) and sigma (s) then you have: G(x) = c * exp ( -0.5 * [(x-m)/s]^2 ) and Integral(-infinity -> infinity) [G(x)] = c * s * sqrt(2*pi) = N where: pi = 3.141592654. In your case Area = N so you get the formula: Area = c * s * sqrt(2*pi) That's for the built-in function, if you want to get the area directly then create your own Gaussian and define it as: G(x) = { c / [ s * sqrt(2*pi) ] } * exp ( -0.5 * [(x-m)/s]^2 ) In this case the constant term c is the area directly. Hope that helps. Stelios. ----- Original Message ----- From: "Saleem, Muhammad" <saleem@SLAC.stanford.edu> To: <roottalk@pcroot.cern.ch> Sent: Wednesday, July 30, 2003 6:51 PM Subject: [ROOT] built in gaussian > Hi > > The builtin "gaus" function, when i fit gives me : > constant > Mean > and width(sigma) > > how do i get the area under the gaussian. > > or why this does not give me the area? > > am i doing something wrong? > > regards > > ---saleem > > >
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