Fit with "gaus" converges but doesn't work

Fatess · October 29, 2021, 1:35pm

Hi, I’m trying to fit an histogram using Fit(“gaus”, “”, “”, xmin, xmax), but even if it converges, it doesn’t work properly, as you ca see in the images (up-left histogram in the image).
The code is the following:

TH1F* hpKDiff = new TH1F("hpKDiff", "hpKDiff", 500, 0, 4);
hpKDiff->Add(hpKInvMassPNNP, hpKInvMassPPNN, 1, -1);
hpKDiff->Fit("gaus", "", "", 0.65, 1.1);

I also tried to change the restiction and the number of bins, but it still doesn’t work.
Another thing I tried is to fit with “gaus” (from the canvas) the down-left histogram, end it does the same thing.
What can be the problem?
Thank you in advance

bellenot · October 29, 2021, 2:11pm

Maybe @moneta can give more details, and maybe I’m wrong, but if I understand correctly the TH1::Fit() documentation:

In order to use the Range option, one must first create a function with the expression to be fitted. For example, if your histogram has a defined range between -4 and 4 and you want to fit a gaussian only in the interval 1 to 3, you can do:
 TF1 *f1 = new TF1("f1", "gaus", 1, 3);
 histo->Fit("f1", "R");

So in your case, can you try something like:

TF1 *f1 = new TF1("f1", "gaus", 0.65, 1.1);
hpKDiff->Fit("f1", "R");

Fatess · October 29, 2021, 2:17pm

I think that is just a different way to do the same think, in fact I tried it but the result is the same.

Dilicus · October 29, 2021, 2:20pm

Hi @Fatess, welcome in the ROOT forum.
It is important to give reasonable starting parameters for the fit.
Try to define the function and set the parameters and then try again the fit.

TF1  * f=new TF1("f","gaus",0.65,1.1);
f->SetParameters(7000,0.9,0.04);
hpKDiff->Fit("f1", "R");

Stefano

Fatess · October 29, 2021, 2:43pm

Thank you for the answer.
Unfortunately, I’ve just tried it but it did’t work, nothing changed.

Dilicus · October 29, 2021, 3:09pm

I just noticed that I wrote f1 here
hpKDiff->Fit("f1", "R");
instead of f
hpKDiff->Fit("f", "R");

Can I ask you how the histogram hpKDiff has been computed, since there are bins with negative value.
Otherwise I think the best way to fit histogram is to use the Likelihood method with the option “L”, but I am not sure if is correct in your case.

Fatess · October 29, 2021, 3:18pm

I noticed it before testing, so it doesn’t work anyway >.<
The histogram has been computed as the difference of this other 2:

with the Add method I wrote in the code above in the post.
Moreover, as I said I tried also with the lower histogram in the post image, right clicking it from the canvas and trying a “gaus” fit, but it did the same thing, and that one is just filled with the Fill method.

Dilicus · October 29, 2021, 3:27pm

You are right, I missed it.

Before do the Add operation call hpKDiff->Sumw2(), otherwise the errors of the bin are not computed correctly.

On the last Histogram the L option should work for sure, set Likelihood as fit method and maybe try to use a narrower range.

Fatess · October 29, 2021, 3:32pm

I already used the Sumw2 method for the two operands, and now I added it for hpKDiff too, but again nothing changed

Dilicus · October 29, 2021, 4:11pm

But even the fit with the Likelihood option on the lower histogram fails, also with the fit range between 0.82 and 0.96?

If also the likelihood fit of the bottom histogram fails, I have really no idea of what is going on

Fatess · October 29, 2021, 4:36pm

Yes, here both are with the Likelihood

Dilicus · October 29, 2021, 4:59pm

Can you share for the last histogram share a file txt with bin center and the bin content together and the histogram binning ?
I will try to do some test on my own.

Fatess · October 29, 2021, 6:37pm

I was trying to share the .C file with the canvas but it says new users can’t share links (I used the upload button on this forum). How can I send that?

Dilicus · October 29, 2021, 7:03pm

Maybe can you try with a google drive or dropbox link

Fatess · October 29, 2021, 7:43pm

It doesn’t work, but I think I can copy and paste it here.

void Test()
{
//=========Macro generated from canvas: test/test
//=========  (Fri Oct 29 20:31:58 2021) by ROOT version 6.22/08
   TCanvas *test = new TCanvas("test", "test",112,135,750,750);
   test->Range(0,0,1,1);
   test->SetFillColor(0);
   test->SetBorderMode(0);
   test->SetBorderSize(2);
   test->SetFrameBorderMode(0);
   
   TH1F *hInvMassDecay__5 = new TH1F("hInvMassDecay__5","Inv. Mass, Pion and Kaon, decay products",500,0.5,1.5);
   hInvMassDecay__5->SetBinContent(78,1);
   hInvMassDecay__5->SetBinContent(90,1);
   hInvMassDecay__5->SetBinContent(91,1);
   hInvMassDecay__5->SetBinContent(93,1);
   hInvMassDecay__5->SetBinContent(96,1);
   hInvMassDecay__5->SetBinContent(100,1);
   hInvMassDecay__5->SetBinContent(101,1);
   hInvMassDecay__5->SetBinContent(102,2);
   hInvMassDecay__5->SetBinContent(103,3);
   hInvMassDecay__5->SetBinContent(104,2);
   hInvMassDecay__5->SetBinContent(105,1);
   hInvMassDecay__5->SetBinContent(106,1);
   hInvMassDecay__5->SetBinContent(107,4);
   hInvMassDecay__5->SetBinContent(108,5);
   hInvMassDecay__5->SetBinContent(109,4);
   hInvMassDecay__5->SetBinContent(110,1);
   hInvMassDecay__5->SetBinContent(111,3);
   hInvMassDecay__5->SetBinContent(112,4);
   hInvMassDecay__5->SetBinContent(113,9);
   hInvMassDecay__5->SetBinContent(114,6);
   hInvMassDecay__5->SetBinContent(115,7);
   hInvMassDecay__5->SetBinContent(116,9);
   hInvMassDecay__5->SetBinContent(117,13);
   hInvMassDecay__5->SetBinContent(118,12);
   hInvMassDecay__5->SetBinContent(119,17);
   hInvMassDecay__5->SetBinContent(120,16);
   hInvMassDecay__5->SetBinContent(121,13);
   hInvMassDecay__5->SetBinContent(122,23);
   hInvMassDecay__5->SetBinContent(123,14);
   hInvMassDecay__5->SetBinContent(124,19);
   hInvMassDecay__5->SetBinContent(125,26);
   hInvMassDecay__5->SetBinContent(126,24);
   hInvMassDecay__5->SetBinContent(127,43);
   hInvMassDecay__5->SetBinContent(128,36);
   hInvMassDecay__5->SetBinContent(129,43);
   hInvMassDecay__5->SetBinContent(130,35);
   hInvMassDecay__5->SetBinContent(131,56);
   hInvMassDecay__5->SetBinContent(132,55);
   hInvMassDecay__5->SetBinContent(133,67);
   hInvMassDecay__5->SetBinContent(134,90);
   hInvMassDecay__5->SetBinContent(135,63);
   hInvMassDecay__5->SetBinContent(136,86);
   hInvMassDecay__5->SetBinContent(137,96);
   hInvMassDecay__5->SetBinContent(138,107);
   hInvMassDecay__5->SetBinContent(139,131);
   hInvMassDecay__5->SetBinContent(140,135);
   hInvMassDecay__5->SetBinContent(141,144);
   hInvMassDecay__5->SetBinContent(142,180);
   hInvMassDecay__5->SetBinContent(143,150);
   hInvMassDecay__5->SetBinContent(144,156);
   hInvMassDecay__5->SetBinContent(145,188);
   hInvMassDecay__5->SetBinContent(146,257);
   hInvMassDecay__5->SetBinContent(147,232);
   hInvMassDecay__5->SetBinContent(148,254);
   hInvMassDecay__5->SetBinContent(149,259);
   hInvMassDecay__5->SetBinContent(150,300);
   hInvMassDecay__5->SetBinContent(151,314);
   hInvMassDecay__5->SetBinContent(152,329);
   hInvMassDecay__5->SetBinContent(153,369);
   hInvMassDecay__5->SetBinContent(154,346);
   hInvMassDecay__5->SetBinContent(155,449);
   hInvMassDecay__5->SetBinContent(156,415);
   hInvMassDecay__5->SetBinContent(157,420);
   hInvMassDecay__5->SetBinContent(158,459);
   hInvMassDecay__5->SetBinContent(159,517);
   hInvMassDecay__5->SetBinContent(160,564);
   hInvMassDecay__5->SetBinContent(161,622);
   hInvMassDecay__5->SetBinContent(162,635);
   hInvMassDecay__5->SetBinContent(163,691);
   hInvMassDecay__5->SetBinContent(164,713);
   hInvMassDecay__5->SetBinContent(165,759);
   hInvMassDecay__5->SetBinContent(166,798);
   hInvMassDecay__5->SetBinContent(167,805);
   hInvMassDecay__5->SetBinContent(168,821);
   hInvMassDecay__5->SetBinContent(169,787);
   hInvMassDecay__5->SetBinContent(170,951);
   hInvMassDecay__5->SetBinContent(171,976);
   hInvMassDecay__5->SetBinContent(172,1058);
   hInvMassDecay__5->SetBinContent(173,1058);
   hInvMassDecay__5->SetBinContent(174,1113);
   hInvMassDecay__5->SetBinContent(175,1137);
   hInvMassDecay__5->SetBinContent(176,1117);
   hInvMassDecay__5->SetBinContent(177,1220);
   hInvMassDecay__5->SetBinContent(178,1220);
   hInvMassDecay__5->SetBinContent(179,1263);
   hInvMassDecay__5->SetBinContent(180,1346);
   hInvMassDecay__5->SetBinContent(181,1283);
   hInvMassDecay__5->SetBinContent(182,1343);
   hInvMassDecay__5->SetBinContent(183,1341);
   hInvMassDecay__5->SetBinContent(184,1404);
   hInvMassDecay__5->SetBinContent(185,1352);
   hInvMassDecay__5->SetBinContent(186,1440);
   hInvMassDecay__5->SetBinContent(187,1570);
   hInvMassDecay__5->SetBinContent(188,1577);
   hInvMassDecay__5->SetBinContent(189,1541);
   hInvMassDecay__5->SetBinContent(190,1576);
   hInvMassDecay__5->SetBinContent(191,1555);
   hInvMassDecay__5->SetBinContent(192,1524);
   hInvMassDecay__5->SetBinContent(193,1518);
   hInvMassDecay__5->SetBinContent(194,1537);
   hInvMassDecay__5->SetBinContent(195,1569);
   hInvMassDecay__5->SetBinContent(196,1620);
   hInvMassDecay__5->SetBinContent(197,1554);
   hInvMassDecay__5->SetBinContent(198,1658);
   hInvMassDecay__5->SetBinContent(199,1611);
   hInvMassDecay__5->SetBinContent(200,1561);
   hInvMassDecay__5->SetBinContent(201,1564);
   hInvMassDecay__5->SetBinContent(202,1583);
   hInvMassDecay__5->SetBinContent(203,1554);
   hInvMassDecay__5->SetBinContent(204,1523);
   hInvMassDecay__5->SetBinContent(205,1579);
   hInvMassDecay__5->SetBinContent(206,1457);
   hInvMassDecay__5->SetBinContent(207,1449);
   hInvMassDecay__5->SetBinContent(208,1437);
   hInvMassDecay__5->SetBinContent(209,1505);
   hInvMassDecay__5->SetBinContent(210,1364);
   hInvMassDecay__5->SetBinContent(211,1295);
   hInvMassDecay__5->SetBinContent(212,1324);
   hInvMassDecay__5->SetBinContent(213,1293);
   hInvMassDecay__5->SetBinContent(214,1231);
   hInvMassDecay__5->SetBinContent(215,1204);
   hInvMassDecay__5->SetBinContent(216,1126);
   hInvMassDecay__5->SetBinContent(217,1170);
   hInvMassDecay__5->SetBinContent(218,1031);
   hInvMassDecay__5->SetBinContent(219,1065);
   hInvMassDecay__5->SetBinContent(220,1035);
   hInvMassDecay__5->SetBinContent(221,1040);
   hInvMassDecay__5->SetBinContent(222,926);
   hInvMassDecay__5->SetBinContent(223,887);
   hInvMassDecay__5->SetBinContent(224,848);
   hInvMassDecay__5->SetBinContent(225,817);
   hInvMassDecay__5->SetBinContent(226,833);
   hInvMassDecay__5->SetBinContent(227,749);
   hInvMassDecay__5->SetBinContent(228,706);
   hInvMassDecay__5->SetBinContent(229,641);
   hInvMassDecay__5->SetBinContent(230,633);
   hInvMassDecay__5->SetBinContent(231,625);
   hInvMassDecay__5->SetBinContent(232,596);
   hInvMassDecay__5->SetBinContent(233,552);
   hInvMassDecay__5->SetBinContent(234,540);
   hInvMassDecay__5->SetBinContent(235,510);
   hInvMassDecay__5->SetBinContent(236,453);
   hInvMassDecay__5->SetBinContent(237,447);
   hInvMassDecay__5->SetBinContent(238,397);
   hInvMassDecay__5->SetBinContent(239,386);
   hInvMassDecay__5->SetBinContent(240,358);
   hInvMassDecay__5->SetBinContent(241,328);
   hInvMassDecay__5->SetBinContent(242,303);
   hInvMassDecay__5->SetBinContent(243,278);
   hInvMassDecay__5->SetBinContent(244,267);
   hInvMassDecay__5->SetBinContent(245,224);
   hInvMassDecay__5->SetBinContent(246,219);
   hInvMassDecay__5->SetBinContent(247,197);
   hInvMassDecay__5->SetBinContent(248,190);
   hInvMassDecay__5->SetBinContent(249,174);
   hInvMassDecay__5->SetBinContent(250,159);
   hInvMassDecay__5->SetBinContent(251,161);
   hInvMassDecay__5->SetBinContent(252,155);
   hInvMassDecay__5->SetBinContent(253,107);
   hInvMassDecay__5->SetBinContent(254,120);
   hInvMassDecay__5->SetBinContent(255,116);
   hInvMassDecay__5->SetBinContent(256,91);
   hInvMassDecay__5->SetBinContent(257,89);
   hInvMassDecay__5->SetBinContent(258,58);
   hInvMassDecay__5->SetBinContent(259,73);
   hInvMassDecay__5->SetBinContent(260,71);
   hInvMassDecay__5->SetBinContent(261,59);
   hInvMassDecay__5->SetBinContent(262,39);
   hInvMassDecay__5->SetBinContent(263,53);
   hInvMassDecay__5->SetBinContent(264,45);
   hInvMassDecay__5->SetBinContent(265,43);
   hInvMassDecay__5->SetBinContent(266,26);
   hInvMassDecay__5->SetBinContent(267,43);
   hInvMassDecay__5->SetBinContent(268,21);
   hInvMassDecay__5->SetBinContent(269,28);
   hInvMassDecay__5->SetBinContent(270,26);
   hInvMassDecay__5->SetBinContent(271,20);
   hInvMassDecay__5->SetBinContent(272,17);
   hInvMassDecay__5->SetBinContent(273,13);
   hInvMassDecay__5->SetBinContent(274,16);
   hInvMassDecay__5->SetBinContent(275,12);
   hInvMassDecay__5->SetBinContent(276,15);
   hInvMassDecay__5->SetBinContent(277,8);
   hInvMassDecay__5->SetBinContent(278,9);
   hInvMassDecay__5->SetBinContent(279,7);
   hInvMassDecay__5->SetBinContent(280,5);
   hInvMassDecay__5->SetBinContent(281,6);
   hInvMassDecay__5->SetBinContent(282,3);
   hInvMassDecay__5->SetBinContent(283,6);
   hInvMassDecay__5->SetBinContent(284,3);
   hInvMassDecay__5->SetBinContent(285,3);
   hInvMassDecay__5->SetBinContent(286,2);
   hInvMassDecay__5->SetBinContent(288,1);
   hInvMassDecay__5->SetBinContent(289,1);
   hInvMassDecay__5->SetBinContent(290,1);
   hInvMassDecay__5->SetBinContent(291,4);
   hInvMassDecay__5->SetBinContent(293,1);
   hInvMassDecay__5->SetBinContent(294,2);
   hInvMassDecay__5->SetBinContent(295,1);
   hInvMassDecay__5->SetEntries(100466);
   
   TPaveStats *ptstats = new TPaveStats(0.78,0.775,0.98,0.935,"brNDC");
   ptstats->SetName("stats");
   ptstats->SetBorderSize(1);
   ptstats->SetFillColor(0);
   ptstats->SetTextAlign(12);
   ptstats->SetTextFont(42);
   TText *ptstats_LaTex = ptstats->AddText("hInvMassDecay");
   ptstats_LaTex->SetTextSize(0.0368);
   ptstats_LaTex = ptstats->AddText("Entries = 100466 ");
   ptstats_LaTex = ptstats->AddText("Mean  = 0.8917");
   ptstats_LaTex = ptstats->AddText("Std Dev   = 0.05016");
   ptstats->SetOptStat(1111);
   ptstats->SetOptFit(0);
   ptstats->Draw();
   hInvMassDecay__5->GetListOfFunctions()->Add(ptstats);
   ptstats->SetParent(hInvMassDecay__5);

   Int_t ci;      // for color index setting
   TColor *color; // for color definition with alpha
   ci = TColor::GetColor("#000099");
   hInvMassDecay__5->SetLineColor(ci);
   hInvMassDecay__5->GetXaxis()->SetLabelFont(42);
   hInvMassDecay__5->GetXaxis()->SetTitleOffset(1);
   hInvMassDecay__5->GetXaxis()->SetTitleFont(42);
   hInvMassDecay__5->GetYaxis()->SetLabelFont(42);
   hInvMassDecay__5->GetYaxis()->SetTitleFont(42);
   hInvMassDecay__5->GetZaxis()->SetLabelFont(42);
   hInvMassDecay__5->GetZaxis()->SetTitleOffset(1);
   hInvMassDecay__5->GetZaxis()->SetTitleFont(42);
   hInvMassDecay__5->Draw("");
   test->Modified();
   test->cd();
   test->SetSelected(test);
}

Wile_E_Coyote · October 30, 2021, 8:27am

No problem here:

root [0] .x Test.C 
root [1] hInvMassDecay__5->Print("base");
TH1.Print Name  = hInvMassDecay__5, Entries= 100466, Total sum= 100466
          Title = Inv. Mass, Pion and Kaon, decay products
          NbinsX= 500, xmin= 0.5, xmax=1.5
root [2] ROOT::Math::MinimizerOptions::SetDefaultMinimizer("Minuit2");
root [3] gStyle->SetOptFit(1)
root [4] hInvMassDecay__5->Fit("gaus", "L", "", 0.65, 1.1);

****************************************
Minimizer is Minuit2 / Migrad
MinFCN                    =      119.353
Chi2                      =      232.755
NDf                       =          222
Edm                       =  6.67897e-08
NCalls                    =           56
Constant                  =      1597.99   +/-   6.177       
Mean                      =     0.891675   +/-   0.00015829  
Sigma                     =    0.0501642   +/-   0.000112075  	 (limited)

Fatess · October 30, 2021, 11:59am

I’ve just found out that it works if I use the .C file, but it doesn’t work if I do the exact same things on the macro that generates it.
About the up-left one, in the macro the fit is just wrong, in the .C file it actually shows the right values, but graphically it’s diplayed as a stright line

Wile_E_Coyote · October 30, 2021, 4:00pm

Attach (“Upload”) your macro and any required data files.

Fatess · October 31, 2021, 3:35am

I’ve just found out the problem, and it was a mistake of mine
I needed to fit another histogram with an exp function, and I defined a TF1 as “exp”, but it seems that a different “exp” is called in “gaus” too, and so it started working differenctly.
Now that I changed the name of my “exp” function it fits perfectly.
Thank you all for the answers!