Thread Subject: Curve Fitting Toolbox

Hay,

I am a newcomer in matlab. I have an exponential equation which needs to be fitted y=a*exp(-bx) - I need a and b. I used the curve fitting toolbox but I am having troubles in setting it up. I choose custom equation and entered some initial value but it looks completely odd. Anyway, I figured out I have to adjust Fit Options for customs... but I have no idea how to play with those values (DiffMinChange, MaxMinChange, ....)! Maybe some one of you!?

I appreciate every help!

Cheers, Katja

> I am a newcomer in matlab. I have an exponential equation which needs to
> be fitted y=a*exp(-bx) - I need a and b. I used the curve fitting toolbox
> but I am having troubles in setting it up. I choose custom equation and
> entered some initial value but it looks completely odd. Anyway, I figured
> out I have to adjust Fit Options for customs... but I have no idea how to
> play with those values (DiffMinChange, MaxMinChange, ....)! Maybe some one
> of you!?

Katja, I suggest you first try the built-in 'exp1' fit type. It's the same
as yours but with no negative sign on the exponent. The toolbox has some
special code to locate good starting values for built-in fit types that it
understands. Sometimes bad initial values are the source of difficulty in
fitting custom equations.

-- Tom

Although I usually do a simple linear fit of log(y) for fitting exponentials,
you can also have a look to the Ezyfit toolbox,
http://www.mathworks.com/matlabcentral/fileexchange/10176
which has a demonstration page where such fit is illustrated, see:
http://www.mathworks.com/matlabcentral/fx_files/10176/3/content/ezyfit/demo/html/efdemo.html
F.


"Katjes " <katja_puder@yahoo.de> wrote in message <ha1eqm$agl$1@fred.mathworks.com>...
> Hay,
>
> I am a newcomer in matlab. I have an exponential equation which needs to be fitted y=a*exp(-bx) - I need a and b. I used the curve fitting toolbox but I am having troubles in setting it up. I choose custom equation and entered some initial value but it looks completely odd. Anyway, I figured out I have to adjust Fit Options for customs... but I have no idea how to play with those values (DiffMinChange, MaxMinChange, ....)! Maybe some one of you!?
>
> I appreciate every help!
>
> Cheers, Katja