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vanhufau

France
4 Posts
Posted - 08/02/2001 :  07:43:43 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
I am trying to fit data with a Hill´s function. I have defined this function, and I can´t find any problem in the expression.
I have 12 parameters, but each time I try to fit, it says :"try to set parameter ... as fixed", and it works only when more than 7 parameters are fixed.
Why?????

My function is :
y=2*mu1*(x^a1-(x*(1.35275-0.36352*x)^2)^(-a1*b1))/(a1*x) + 2*mu2*(x^a2-(x*(1.35275-0.36352*x)^2)^(-a2*b2))/(a2*x) + 2*mu3*(x^a3-(x*(1.35275-0.36352*x)^2)^(-a3*b3))/(a3*x) + 2*mu4*(x^a4-(x*(1.35275-0.36352*x)^2)^(-a4*b4))/(a4*x)

vanhufau

France
4 Posts
Posted - 08/06/2001 :  03:55:41 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
The problem was with my initial values...
Thank you!
Now I have another problem : It doesn´t return me any value for the CHI^2...
?

Faustine
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rtoomey

USA
184 Posts
Posted - 08/07/2001 :  5:06:55 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Faustine,

An empty chi-sqr value might indicate that the initial parameter guesses were inadequate and have caused the fit to wander off (i.e. it isn't converging). Fitting with the Levenburg-Marquardt algorithm (as you are in our fitter in most cases) requires fairly reasonable initial parameter estimates.

In a situation like this, it would be best if you sent the data to Technical Support along with your initial parameter estimates. The function can then be scrutinized and suggestions can be made as to how to improve it and make the fit work.

- rtoomey
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