fitting unsorted data

From: Voropaev S.G. (voropaev@inp.nsk.su)
Date: Fri Aug 29 1997 - 07:08:38 MEST


Hello,
I have new problem with fitting unsorted data. I am trying to fit unsorted
array X,Y to line. I use object TGraph. Parameters of line is well, but error
is very strange. 
Test macro with only 5 data point. (In real life number of data point is
larger, but problem still exist):

{
	float Xarr[5];
	float Yarr[5];
	Xarr[0] = 0.062893; Yarr[0] = 0.584000;
	Xarr[1] = 0.000000; Yarr[1] = 0.646000;
	Xarr[2] = 0.062893; Yarr[2] = 0.572000;
	Xarr[3] = 0.125786; Yarr[3] = 0.497000;
	Xarr[4] = 0.125786; Yarr[4] = 0.533000;
//	Xarr[5] = 0.000000; Yarr[5] = 0.626000;
//	Xarr[6] = 0.125786; Yarr[6] = 0.529000;
//	Xarr[7] = 0.251572; Yarr[7] = 0.432000;
//	Xarr[8] = 0.125786; Yarr[8] = 0.534000;
//	Xarr[9] = 0.251572; Yarr[9] = 0.441000;
//	Xarr[10] = 0.188679; Yarr[10] = 0.480000;
//	Xarr[11] = 0.000000; Yarr[11] = 0.627000;
//	Xarr[12] = 0.377358; Yarr[12] = 0.338000;
//	Xarr[13] = 0.188679; Yarr[13] = 0.467000;
//	Xarr[14] = 0.251572; Yarr[14] = 0.405000;
//	Xarr[15] = 0.000000; Yarr[15] = 0.646000;
//	Xarr[16] = 0.503145; Yarr[16] = 0.252000;
//	Xarr[17] = 0.251572; Yarr[17] = 0.366000;

	c1 = new TCanvas("c1","proba");

	TF1 *f1 = new TF1("f1","pol1");
	f1->SetRange(0,0.15);	
	
	TGraph *tg = new TGraph(5, Xarr, Yarr);
	tg->Fit("f1","R","AP");
	tg->Draw("A*");	
}

Result of fitting:	p0 = 0.64 +- 0.85 !,  p1 = -1.04 +- 9.5 !!!

Where is error? It is my error or not?

I use ROOT 1.02/00 1 august 1997 ( Valery Fine version) on Windows NT
	
			With best regards Voropaev S.G.



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