R help - Jun 2011 - logistic growth model

I want to Fit a logistic growth model for
y = k *eb0+b1(age)/1 + eb0+b1(age), can some one help on how to get the 
initial coefficients b0 and b1? I need to estimate in order to do the 
regression analysis. When I run using b0=0.5 and b1=3.4818, I get the 
following error

397443.8 :  0.5 3.4818
Error in nls(Height ~ k * exp(b1 + b2 * Age)/(1 + exp(b1 + b2 * Age)),  :
   singular gradient
"please tell me what is wrong with my initials values, and how to get 
the initial values"

HI I want to Fit a logistic growth model for this data set where
y = k*exp(b0+b1(age))/1 + exp(bo+b1(age)),
start = list(b0 = 3, b1 = 3.5 ),
                       trace = TRUE)

I need to find the initial valued for b0 and b1. K =3. When I run using b0=3
abd b1 = 3.5, or any number I get the following error


397448.4 :  3.0 3.5 
Error in numericDeriv(form[[3L]], names(ind), env) : 
  Missing value or an infinity produced when evaluating the model. Can
anyone help me how to get the initial values? what does this error msg
implies?
> 

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Write the growth formula in an R script.
Define initial par values.
Input the size and age data.
Plot the size and age data as points.
Plot the growth model with the initial par values as a line.
Play with the initial par values until you see a good agreement between the
model (the line) and the data (the points).
Optimise.
Re-plot.
Plot a residual histogram.
Plot a residual scatterplot.
Plot a Q-Q residual plot.

HTH

Rub?n
____________________________________________________________________________________

Dr. Rub?n Roa-Ureta
AZTI - Tecnalia / Marine Research Unit
Txatxarramendi Ugartea z/g
48395 Sukarrieta (Bizkaia)
SPAIN


> -----Mensaje original-----
> De: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] En nombre de Renalda
> Enviado el: s?bado, 04 de junio de 2011 6:17
> Para: r-help at r-project.org
> Asunto: [R] logistic growth model
> 
> I want to Fit a logistic growth model for y = k 
> *eb0+b1(age)/1 + eb0+b1(age), can some one help on how to get 
> the initial coefficients b0 and b1? I need to estimate in 
> order to do the regression analysis. When I run using b0=0.5 
> and b1=3.4818, I get the following error
> 
> 397443.8 :  0.5 3.4818
> Error in nls(Height ~ k * exp(b1 + b2 * Age)/(1 + exp(b1 + b2 
> * Age)),  :
>    singular gradient
> "please tell me what is wrong with my initials values, and 
> how to get the initial values"
> 
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