Three exponential fit with curve fitting toolbox

Hi,

I'm using the Curve Fitting Toolbox in MatLab and I don't see a way to do a three term exponential fit. Is there a way I can do it? If the custom equation can be helpful, how can I use it for that kind of problem? And why MatLab Curve Fitting Toolbox doesn't offer the three term exponential fit?

Best regards,

Kateri.

Answer 1

[Asad (Mehrzad) Khoddam]

In the toolbox, there is an option to define a custom model.

https://www.mathworks.com/help/curvefit/custom-nonlinear-models.html

[Katéri Tremblay]

Thanks for the reply! I tried to make an three term exponential fit by customizing an equation, but it doesn't work either. I obtain this:

[Asad (Mehrzad) Khoddam]

If you have any ideas about the cofficients, you can set a lower and upper values for them instead of -Inf and Inf

Answer 2

[John D'Errico]

You an always define your own custom model using fittype. Nothing stops you. But multiple term sums of exponential fits are often difficult to estimate well. Personally, if I was writing the curvefitting toolbox, I would leave it out too, just because most people lack good enough data or good enough starting values. So I would force the person who really wants to do that, to also know enough about what they are doing to have even a remote chance of success.

But nothing stops you from doing as you wish. As I said though, good data and good starting vaues will be crucial. If you allow it to choose its own random starting values, then expect possibly total crap for a result.