nonlinear fit with fixed starting point<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
Hi,
As I am exploring the functions of JMP more and more, here is one I haven't been able to crack yet...
I am trying to fit an exponential 3 parameter function through some datapoints that report yield in response to a certain dosage of products. I can get the nonlinear fit of the model, but what I would like to be able to do is to force the fit to start at the response variable for the dosage zero.
So if the model is y=a+b*exp(c*x), where x=0. I guess my real question is if you can restrict the model and force it through a given point.
Any ideas on how to get this to work?
Re: nonlinear fit with fixed starting pointAh,
It's always so simple when someone explains it... Great tip, thanks!
Re: nonlinear fit with fixed starting pointIf I understand you correctly, you want your model f(x) = a+b*exp(c*x) to agree exactly with the data value you have for x=0, call it y0. Then for x=0 you want
y0 = f(0) = a + b
Solving, say, for a, we get a = y0-b. In other words you can reduce the number of parameters from 3 to 2 by fitting the model for b and c (and get a=y0-b).
f(x) = y0-b + b*exp(c*x) which will automatically satisfy f(0) = y0.
There is also an option in the nonlinear platform dialog to specify constraints on the parameters but the direct route above is the simplest way to approach your problem as I understand it.
Michael
