I am trying to model a function of observed data in JAGS. For example

   x[i,t]/phi ~ dpois(mu)

where x is observed and phi is a parameter in the model. I can't seem to get this working and am wondering if I may be missing some larger issue in JAGS. I am pretty sure you can't have any transformation of the data in the model statement and need to use a data block but that doesn't seem to be dynamic, thus not allowing for me to fit phi.

Any ideas?

  • $\begingroup$ Can you please add the formulas for your model? $\endgroup$
    – teucer
    Commented Nov 7, 2011 at 10:23
  • $\begingroup$ it is a pretty convoluted model and I think it will complicate the question. Is my question unclear? $\endgroup$
    – scottyaz
    Commented Nov 7, 2011 at 11:24
  • 3
    $\begingroup$ I believe you cannot fit the model that you have described: to my knowledge in JAGS you are not able to scale a distribution. What distribution $\phi$ is supposed to follow? If it is simple enough, you can derive the distribution of $x$ directly and sample from it. $\endgroup$
    – teucer
    Commented Nov 7, 2011 at 11:48

1 Answer 1


So here the answer from the comment:

I believe there is no way to directly sample from a scaled distribution in JAGS, except the cases where the scaled distribution can be expressed as a standard distribution.

This being said there is trick, the so called "zero trick", but I have not used it so far. It might be useful in your case...


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