Non-linear curve fitting fail to converge
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I'm using the curve fitting toolbox to fit the data to a function: y = a*(1-b*exp(-c*x)-(1-b)*exp(-d*x)).
For a certain group of data, the fitting fails to converge. (Algorithm: Trust region. The boudaries set for the parameters are: a: 0.25 - Inf, b: 0-1, c: 0-Inf, d: 0-Inf)
For some groups of data, the fitting can converge but the standard error of the fitted parameters are very big.
I have a feeling that this is because my function is overparameterised, but I can't prove.
Thanks a lot in advance.