Solution for Trap 66 (Postcondition violated)

WinBUGS is a widely used free software program within health-economics. It allows for Bayesian statistical modelling, using Gibbs sampling. (Hence the name: the Windows version of Bayesian inference Using Gibbs Sampling). One of the drawbacks of WinBUGS is the notoriously uninformative error messages you can receive. While Google is usually a Fountain of Knowledge on solving errors, where WinBUGS is concerned it often only serves up other people asking the same question, and hardly any answers. This post is about one error message that I found, the solution that’s sometimes offered which I think only partly solves the problem and the solution I found which solves it completely.

The error message itself is “Trap 66 (postcondition violated)”. Variance priors have been identified as the culprits. The suggested solutions I could find (for example here, here and here) all point towards those priors being too big. The standard advice is then to reduce the priors (for example from dunif(0,100) to dunif(0,10)) and rerun it. This usually solves the problem.

However, to me, this doesn’t make a whole lot of sense theoretically. And, in a rare case of the two aligning, it also didn’t solve my problems in practice. I have been performing a simulation study, in which I have about 8,000 similar, but different data sets (8 scenarios, 1000 repetitions in each). They all represent mixed treatment comparisons (MTC), which are analyzed by WinBUGS. I used SAS to create the data, send it to WinBUGS and collect and analyse the outcomes. When I started the random effects MTC, the “Trap 66 (postcondition violated)” popped up around dataset 45. Making the priors smaller, as suggested, solved the problem for this particular dataset, but it came back on data set 95. The funny thing is that making the priors higher also solved the problem for the original dataset, but once again the same problem arose at a different data set (this time number 16).

Whenever I tried to recreate the problem, it would give the same error message at the exact same point in time, even though it’s a random sampler. From this it seems to be that the reason why the suggested solution works for one data is that the generated ‘chains’, as they are called in WinBUGS, are the same with the same priors and initial values. Defining a smaller prior will give a different chain which is likely not to cause problems. But so will a larger prior or a different initial value. However, it didn’t really solve the problem.

The solution I have found to work for all 8,000 data sets is to not look at the maximum value of the prior, but at the minimum value. The prior that is given for a standard error usually looks something like dunif(0,X). In my case, I did an extra step, with a prior on a variable called tau, for which I specify a uniform prior. The precision (one divided by the variance) that goes into the link function is defined by

prec <- pow(tau,-2)

This does not make any difference for the problem or the solution. My hypothesis is that when Trap 66 comes up, the chain generates a tau (or standard error, if that’s what you modelled directly) equal to 0, which resolves into a precision equal to 1 divided by 0, or infinity. The solution is to let the prior not start at 0, but at a small epsilon. I used dunif(0.001,10), which solved all my problems.

This solution is related to a different problem I once had when programming a probability. I mistakenly used a dunif(0,1) prior. Every now and then the chain will generate exactly 0 or 1, which does not sit well with the binomial link function. The error message is different (“Undefined real result”), but the solution is again to use a prior that does not include the extremes. In my case, using a flat dbeta instead (which I should have done to begin with) solved that problem.

Any suggestions and comments are much appreciated. You can download WinBUGS, the free immortality key and lots of examples from the BUGS project website here. It also contains a list of common error messages and their solutions. Not Trap 66, obviously.