When fitting equivalent circuit models to Electrochemical Impedance Spectroscopy Data, Gamry utilizes a downhill simplex method to optimize the fit parameters. The simplex method was first described by Nelder and Mead. They described the simplex method as a method which adapts itself to the local landscape and contracts to a final minimum. One advantage of the simplex method over other minimization techniques is that it does not require derivatives, just function evaluations.
Nelder, J. A. and Mead, R. "A Simplex Method for Function Minimization." Computer Journal 7, 308-313, 1965.
Goodness of Fit
Goodness of fit is Chi Squared. Chi squared is the calculated sum of the weighted residuals. Defined as the following:
ChiSq = Squared Sum of weighted Residuals
Weighting calculated as follows based on user input
Custom weighting can be used when weights are entered manually into the Zsig column of the data file using an ASCII editor.