On Apr 10, 2012, at 4:03 PM, nerak13 wrote:
Hi,
I've got the following data:
x<-c(1,3,5,7)
y<-c(37.98,11.68,3.65,3.93)
penetrationks28<-dataframe(x=x,y=y)
now I need to fit a non linear function so I did:
fit <- nls(y ~ I(a+b*exp(1)^(-c * x)), data = penetrationks28, start =
list(a=0,b = 1,c=1), trace = T)
The error message I get is:
Error in nls(y ~ I(a + b * exp(1)^(-c * x)), data = penetrationks28,
start =
list(a = 0, :
singular gradient
I've tried to change the startervalues but it always gives the same
error
I've also tried the following adjustment hoping that the c value
would be
negative:
fit <- nls(y ~ I(a+b*exp(1)^(c * x)), data = penetrationks28, start
= list(a
= 1,b = 1,c=1), trace = T)
but then the error message is:
Error in nls(y ~ I(a + b * exp(1)^(c * x)), data = penetrationks28,
start =
list(a = 1, :
number of iterations exceeded maximum of 50
What can I do ?
?nls
"Warning:
Do not use nls on artificial "zero-residual" data."
Get more data. (with 3 coefficients and 4 data points you have created
a zero-residual situation even if it's not "artificial".)
--
David Winsemius, MD
West Hartford, CT
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