Hi Alexander, I tried your small program and I obtain the following result of 0.3 as expected using ROOT from CVS of one week ago on RH7.2 : root [0] TH1F* h1=new TH1F("h1","test",10,1,10) root [1] h1->SetBinContent(3,1) root [2] h1->SetBinContent(7,2) root [3] h1->Fit("pol0","W") FCN=4.1 FROM MIGRAD STATUS=CONVERGED 11 CALLS 12 TOTAL EDM=1.0137e-24 STRATEGY= 1 ERROR MATRIX ACCURATE EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 3.00000e-01 3.16228e-01 -0.00000e+00 6.36774e-12 <TCanvas::MakeDefCanvas>: created default TCanvas with name c1 (Int_t)0 Nicolas Alexander Dietz wrote: >Hello Rooters, > >as I know the chi-square method, used to estimate parameters, goes like >this: > > >chisquare=sum_{i=1}^N (x_i - y_i)^2/e_i > > >with x_i being the value from a spectrum and y_i being the theoretical >function depending on one or more parameters. >To get the best-fitting values for the parameters this function must be >minimized. >Suppose to fit a constant y with fit-option 'w' (setting all errors to 1). >Then the chisquare-value should be calculated like this: > > >chisquare=sum_{i=1}^N (x_i - y)^2 > > >It is simple to calculate y for which this value is minimal: > >y=( sum_{i=1}^n x_i ) /n > >so y is simple the mean value of the data in the spectrum. > >I checked this with the following program: > > > TH1F* h1=new TH1F("h1","test",10,1,10); > h1->SetBinContent(3,1); > h1->SetBinContent(7,2); > h1->Fit("pol0","W"); > >but what I get is a best-value of 1.5 instead of 0.3! >It seems that ROOT ignores every channel with zero-content. > >Is ROOT ignoring channels with zero-contente and why? >What can I do that ROOT gives the correct value? > >Cheers > > Alexander Dietz > > > > -- :-------------------------------: : Nicolas de SEREVILLE : : C.S.N.S.M. : : Batiment 104 : : 91405 ORSAY Campus : : Tel : (+ 33) (0)1 69 15 52 29 : : Fax : (+ 33) (0)1 69 15 50 08 : :-------------------------------:
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