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Re: growth performance index, vB curves


William Silvert <[log in to unmask]>


[log in to unmask]


Fri, 16 Jun 2000 02:27:43 -0700





text/plain (1 lines)

I feel that a word of caution is called for in
comparing von Bertalanfy growth curves. Most programs
for fitting these curves give parameter estimates
without confidence limits, and of course the
calculation of confidence limits for nonlinear models
is something that most analysts try to avoid. However
I have found that these ranges can be very broad, and
in particular I have frequently found that the
confidence range for L infinity can include infinity
-- even though the data may seem to approach an
asymptote, this is not significant in a statistical
sense, and a straight line provides an acceptable fit
to the data. Also, some fitting techniques use
incorrect transformations, and will not provide
correct fits if one of the fish in the sample is
larger than the mean L infinity.

Bill Silvert

--- Eduardo Sendra <[log in to unmask]> wrote:
> Dear Listers:
> We are trying to compare growth curves from
> several populations of
> Argentine silversides from nearby localities
> (lakes). We have estimates of von
> Bertalanfy's parameteres for all of them and would
> like to compare overall
> growth or growth performance for the stocks...

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