Setting step size in simple TH2 fitting

ROOT
1

I cannot find an answer in this form or in any of the examples:

Is there any way to set a step size while performing simple TH2 fitting? In FitPanel there is a “step” field for each parameter, but if I correctly understand the sources, this is only used for setting minimum and maximum for the parameter…

2

Hello,
step size can be set by using either the fit panel or by setting directly in TF1 via TF1::SetParError or TF1::SetParErrors.
If the value set is zero, a default step size will be used (typically 10% of the parameter value)

Best Regards

Lorenzo

3

That’s what I tried at the beginning:

	psf_fun->SetParameters(1, 1, 18, 18.4, -1.17536e-02, -2.95840e-02, 9.05276e-03,1.52569e-02, 2.51531e-03);	
	psf_fun->SetParError(1, 10);
	prof->Fit(psf_fun, "W");

Then I get:

 FCN=366.649 FROM MIGRAD    STATUS=CONVERGED      75 CALLS          76 TOTAL
                     EDM=0    STRATEGY= 1  ERROR MATRIX UNCERTAINTY 100.0 per cent
  EXT PARAMETER                APPROXIMATE        STEP         FIRST
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE
   1  p0           1.00000e+00   1.41421e+00   0.00000e+00   0.00000e+00
   2  p1           1.00000e+00   1.41421e+00   0.00000e+00   0.00000e+00

the displayed step size is 0, and during the fit I can see that it starts changing the p1 by 0.3, not by 10 as I requested…

I’ll try to create a working example soon.

4

OK, the modified tutorial example.

The line
f2->SetParError(0, 0.01);

does not seem to have any effect on the step size…
fit2.C (1.37 KB)

5

Hi,

you can see the initail step size only if you use the verbose option (“V”).
Here is what I get by turning on this option

 **********
 **    1 **SET PRINT           2
 **********
 PARAMETER DEFINITIONS:
    NO.   NAME         VALUE      STEP SIZE      LIMITS
     1 p0           4.00000e+02  1.00000e-02     no limits
     2 p1          -3.00000e+00  9.00000e-01     no limits
     3 p2           3.00000e+00  9.00000e-01     no limits
     4 p3          -3.00000e+00  9.00000e-01     no limits
     5 p4           3.00000e+00  9.00000e-01     no limits

so you have 0.01 as expected.
However, which ROOT version are you using ? Maybe with aversion earlier than 5.22, this probably did not work

Lorenzo

6

I’ve got root 5.22/00. Here is my output :confused: :

root [0] .x fit2.C
100
400
 **********
 **    1 **SET PRINT           2
 **********
 PARAMETER DEFINITIONS:
    NO.   NAME         VALUE      STEP SIZE      LIMITS
     1 p0           4.00000e+02  1.20000e+02     no limits
     2 p1          -3.00000e+00  9.00000e-01     no limits
     3 p2           3.00000e+00  9.00000e-01     no limits
     4 p3          -3.00000e+00  9.00000e-01     no limits
     5 p4           3.00000e+00  9.00000e-01     no limits

So obviously something is not working. Maybe I should update…

7

OK, after update the error changes the step size. However, the initial step size is not the real step size - just proportional. Is that intentional?

   1  p0           0.00000e+00   1.00000e+00
   2  p1           0.00000e+00   1.00000e+00

and than first changes of those parameters are:

0.01 0
-0.01 0
0.1 0
-0.1 0