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srira

India
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Posted - 09/24/2005 :  1:17:52 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Origin Version (Select Help-->About Origin): 6.1
Operating System:XP
Hi

I would like to know about the chi sq values.For eg, I fit a curve using NLSF with 4 parameters, so the degrees of freedom is 4. From the chi sq table, the value varies between 4.11(prob.=.25) to 17.73 (prob.=.0005).Hence reduced chi sq. (one gets from origin fitting) varies between 1.03 to 4.4325. Can this be taken as the limits for curve fitting with 4 parameters?What would be the optimum value of chi sq. one has to look for?

Sometime though there are 4 parameters, the optimization is done by fixing any 2 paramaters.Can this be taken as usage of only 2 degrees of freedom?


Thanks!

Mike Buess

USA
3037 Posts
Posted - 09/24/2005 :  3:39:02 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
The number of degrees of freedom is defined as the number of data points minus the number of fitting parameters. I don't believe that fixed parameters are included in the parameter count but am not entirely sure. If you have many more data points than parameters it doesn't really matter.

Help > Origin > Curve Fitting > The Nonlinear Least Square Fitter > Before you start: The Chi-Square Minimization

Mike Buess
Origin WebRing Member
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