Hi,
I have a problem fitting the following Weibull Model to a set of data.
The model is this one: a-b*exp(-c*x^d)
If I fitted the model with CurveExpert I can find a very nice set of coefficients which create a curve very close to my data, but when I use the nls.lm function in R I can't obtain the same result.
My data are these:
X     Y
15    13
50    13
75    9
90    4

With the commercial software I obtain the following coefficients:
Weibull Model: y=a-b*exp(-c*x^d)
Coefficient Data:
a =          1.31636909714E+001
b =          7.61325570579E+002
c =          2.82150000991E+002
d =          -9.23838785044E-001

For fitting the Levenberg-Marquardt in R I'm using the following lines:
pS<-list(a=1,b=1,c=1,d=1)
model<-function(pS,xx){pS$a-pS$b*exp(-pS$c*xx^-pS$d)}
resid<-function(observed,pS,xx){observed-model(pS,xx)}
lin<-nls.lm(pS,resid,observed=Y,xx=X)

Why I can't obtain the same results?
Many thanks in advance,
Fabio


Mr. Fabio Veronesi
Ph.D. student

Cranfield University
e-mail: f.veronesi@cranfield.ac.uk

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