On 20/01/12 06:38, arivald wrote:
Hello there,
I am trying to fit an exponential model using nls to some data.
#data
t<- c(0,15,30,60,90,120,240,360,480)
var<- c(0.36,9.72,15.50,23.50,31.44,40.66,59.81,73.11,81.65)
df<- data.frame(t, var)
# model
# var ~ a+b*(1-exp(-k*t))
# I'm looking for values of a,b and k
# formula
# mod<- nls(formula = var ~ a+b *(1-exp((-k)*t)), start=list(a=0, b=10,
k=0), trace=T)
# It fails and I get the following error -
Error in nlsModel(formula, mf, start, wts) :
singular gradient matrix at initial parameter estimates
I was trying different methods and looking for some solutions but nothing is
working...
Any suggestions how I can fix this? I appreciated any help
Thanks in advance.
Try different starting values. Yours are out to lunch. Look at:
plot(function(t){10*(1-exp(-t))},xlim=c(0,500),xlab="t",ylab="var")
This goes to hell in a handcart at about t=6 and looks nothing like
plot(t,var)
I found that
mod <- nls(formula = var ~ a + b *(1-exp((-k)*t)),
start=list(a=0,b=80, k=0.01))
works OK. Then doing
ccc <- coef(mod)
plot(function(t){ccc[1]
+ccc[2]*(1-exp(-ccc[3]*t))},xlim=c(0,500),xlab="t",ylab="var")
points(t,var)
indicates a fairly reasonable fit. One wonders however about the
validity of the model
considering the lack of fit at t=0.
R has great and easy-to-use graphics capabilities. Use them!
cheers,
Rolf Turner
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