[R] non-finite finite-difference value[]

From: P. Olsson <olsson1_at_gmail.com>
Date: Wed 25 Jan 2006 - 06:30:05 EST


Dear R-helpers,

running a zeroinflated model of the following type: zinb = zeroinfl(count=response ~., x = ~ . - response, z = ~. - response, dist = "negbin", data = t.data, trace = TRUE) generates the following message:

Zero-Inflated Count Model
Using logit to model zero vs non-zero
Using Negative Binomial for counts
dependent variable y:
Y
  0 1 2 3
359 52 7 3
generating start values...done
MLE begins...
initial value 262.883959
error in optim(fn = llhfunc, par = stval, method = method, control = control, :

        non-finite finite-difference value [3]

Short version of my question:
what information does non-finite finite-difference value [3] give?

Extended version of my question:
Since zeroinfl() calls optim(), I assume, that an infinite value is generated during the iteration steps of the optimizing algorithm. So the value[3] gives me a hint to the step, when it happens? Or expressed in a different way, are my data structured in a way, that finding the maximum in the response surface is highly sensitive of the initial value, used for the algorithm?
And if this is correct, is there an easy way in R, to set another starting value that the one, which is generated automatically in zeroinfl() (I mean, just from the technical point of view, without considering the theoretical thrill, which will be my next problem, since it is not at all familiar to me.).
Or last questions: how can I extract the function, generated by zeroinfl() and passed to optim()? Is there a possibility to visualize it, so that I get an idea, what is going on?

Any hint, which will help me to dig further in the right direction is more than welcome.
Thank you in advance.
Kind regards,
P. Olsson

PS: Sorry, I tried, but I did not manage to set up an example which generates the above mentioned message.
PPS: I am running R 2.2.1 under windows.

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