Is plotting the initial guess a prerequisite to get a decent fitting curve?

The Graph Now button is simply a way to show you the curve you would get from the initial guesses entered into the coefficient list. The same initial guesses should give the same fit with or without the Graph Now button.

I assume you are fitting to a user-defined function.

What sort of initial guess do you provide in the case where you get a bad fit? Be sure your coefficient wave is double precision.

Yes, the fitting function is user-defined. But the building function, fExpGauss, is defined in <Multi-peak fitting 2.0> . 

Thank you for your reminder about the coefficient wave. I tried to reset the W_coef wave before every fitting, and the weird thing disappeared.

I think what happened before was the W_coef stores the fitting results from the previous data set, which can be far away from the "correct" initial guess for the current dataset. And it will cause the problem that I described before. 

Appreciate your help!

Note that fExpGauss() is included in an obsolete part of Multipeak Fitting, which will not be loaded together with the Multipeak Fitting package anymore in Igor versions >= 9. The procedure file where this is in (Peak Functions.ipf) is still shipped with Igor 9, though. To keep using it you need to add the following line into the procedure window of each experiment you want to have it:

I rather recommend that you recreate this function with a distinct name in your own code and not rely on the existence of this old stuff anymore. It is just a very short piece of code. Or even better, write a proper implementation of the ExpModGauss function using the newer(?) erfc() function.

Glad I could help.

Looks like your "bad" fit above found a local minimum in chi-square. Some fit functions require better guesses than others.

Thanks for sharing this detailed information, Chozo. I am using version8, but I will check the erfc function.