TF1::FixParameter

From: Thomas Lauf <thl_at_hll.mpg.de>
Date: Mon, 17 Dec 2007 11:39:09 +0100


Hello,

I do not quite understand how the function TF1::FixParameter works. When I run the following macro,

{

   TH1* Hist = new TH1I( "Hist", "Hist", 21, -10.5, +10.5 );    Hist->FillRandom( "gaus" );                                                                                 

   TF1* Fit = new TF1( "myfit", "gaus" );    Fit->FixParameter( 1, 0.0 );

   Hist->Fit( Fit );
}

where I fit a random gaussian distribiution with a function whose mean I fixed at 0.0, I get e.g. the following result:

  FCN=3.83732 FROM MIGRAD STATUS=CONVERGED 41 CALLS 42 TOTAL

                      EDM=1.02145e-10    STRATEGY= 1      ERROR MATRIX 
ACCURATE
   EXT PARAMETER                                   STEP         FIRST
   NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE
    1  Constant     2.00922e+03   3.46544e+01   3.05267e-02  -2.58214e-07
    2  Mean        -9.80000e-03     fixed
    3  Sigma        9.92018e-01   9.78696e-03   2.89142e-06  -5.29594e-03

Ok, the mean is close to 0.0 and declared 'fixed', but why is it not exactly 0.0?

Thanks for any clarifications

Thomas

--
Thomas Lauf
MPE - MPI Halbleiterlabor
E-Mail: thl_at_hll.mpg.de
Received on Mon Dec 17 2007 - 11:39:38 CET

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