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mej
Albania
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 Posted - 11/02/2004 : 09:20:27 AM
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Origin Version : 6.1 Pro
Operating System: wind 2000
Hi everybody,
I want to fit two processes with the following equations:
F1=ainf+(a1*exp(a*x)+a2*exp(-a*x))*exp(-b*x/2) with condition a1+a2=-ainf
and
F2=ainf*(1-K*(1/h1*exp(h1*x)-1/h2*exp(h2*x))) with condition 1/K=1/h1+1/h2
First, how to give to origin the second condition
and my big problem  is that the error in parameters (value+/- error) h1 (4.5+/-1.2), K (~-14+/-6), a1 (-3+/-10), and a2 (-3+/-10)
are big despite the experimental curves are well fitted by these expressions
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easwar
USA
1965 Posts |
Posted - 11/02/2004 : 09:29:47 AM
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Hi,
Are you looking for how to simultaneously fit two datasets with two different equations that share parameters? You can do that in the NLSF tool by specifying two dependent variables, which in your case are F1 and F2, with independent variable being x, and the parameters being ainf, a1, a, a2, b, K, h1, and h2.
As for the second condition (1/K=1/h1+1/h2), that is a nonlinear constraint and currently Origin supports only linear constraints for parameters.
As for the error, even if the fit curve defines the data very well, the error on the parameters could be large due to other reasons. One such reason could be that you have too many parameters in your equation (over-parametrized equation), in which case the chi-sq surface is rather broad and so the minimum in the parameter space is rather broad leading to large parameter errors.
Easwar
OriginLab
Edited by - easwar on 11/02/2004 09:30:49 AM |
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Fitting and error parameters