Matlab fitting error using lsqcurvefit
I'm developing code to fit the Gompertz equation to a bacterial growth curve and am practicing with some example data provided at the following website:
http://www.math.tamu.edu/~phoward/m442/ia3sol.pdf.
According to this code the fit should almost match the data (graph given at above webpage, page 3). However when I run the code the actual data plots correctly but the lsqcurve fit fits very poorly and gives the following message:
Local minimum possible.
lsqcurvefit stopped because the size of the current step is less than
the default value of the step size tolerance.
Is there anything I am doing wrong?
Thank you for your time,
Laura
The problem lies in the linked document.
The Gompertz function is parametrized the following way:
%with parameters p(1) = K and p(2) = initial population
%p(3) = r.
V = p(1).*(p(2)/p(1)).^exp(-p(3)*t);
However, the initial parameters for the curve fitting are given for a different ordering of parameters in p vector ([r, K, p0] instead of [K, p0, r]). Moreover, the result vector is also messed up in the document.
By changing p0 to this [1000, 3.93, 0.01] the curve fitting will converge and you will get a nice fit:
