Jeremy:
Don't be silly. The model is overdetermined -- a and b tradeoff with each
other. e.g. for any solution (a,b), (a/k^m,b+m) for any m is also a
solution, where k = const (assuming I have correctly interpreted the model,
of course).
-- Bert
On Wed, Dec 19, 2012 at 11:59 AM, Jeremy Miles jeremy.mi...@gmail.comwrote:
Could you provide the code that you're running, so we can see what
you're trying to do? Even better would be a repeatable example.
Jeremy
On 19 December 2012 09:42, Yann Labou yann.la...@outlook.com wrote:
Hey all,
I'm trying to fit a non-linear model y ~ a * constant ^ b * x ^ c and
estimates the paramaters a, b and c.
Using the nls function, I'm getting following error message:
Error in nlsModel(formula, mf, start, wts) :
singular gradient matrix at initial parameter estimates
If I logarithmize the whole equation log(y) ~ log (a) + b *
log(constant) +
c * log(x) and fit the equation with lm, I get only NAs estimates for the
second term on the right side:
Coefficients: (1 not defined because of singularities)
Do you have any hints on how to fit this equation or any alternatives to
nls?
Thanks,
Yann
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--
Bert Gunter
Genentech Nonclinical Biostatistics
Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
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