A word of caution on this request - a number of years
ago I was involved with a project based on vB growth
curves and I looked at a lot of different fitting
programs, none of which were satisfactory. The most
common problem is the implicit assumption of an
inappropriate error distribution. Many of the
algorithms were unstable, and as is often the case
with the use of fancy optimisation methods for solving
simple nonlinear problems, you risk finding local
minima.

For nonlinear problems involving just two or three
parameters it is more reliable, and usually faster, to
do the optimisation yourself, as described in Silvert,
W.  1979.  Practical curve fitting.  Limnol. Oceanogr.
24:767-773. You can easily do this with an Excel
spreadsheet, and include a graph so that you can see
how well the curve fits the data as you improve the
fit.

Bill Silvert

--- Corrado Costa <[log in to unmask]> wrote:
> Dear all,
> I'm looking for some biological tools for excell on
> von Bertalanffy equation
> and on Bhattacharya modal progressions.
> Could anyone help me?
> Thank you in advance
> Davide Cascione


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