Sample from posterior predictive after fixing latent varaibles

Questions
1

Hi pymc community,
I have a latent variable mixture model, where a number of rater’s judgments on a particular trail are assigned as either “real” or “contaminate”. The number of possible “real” judgments (z) is latent categorical variable inferred during sampling.

After I have fit the model, and generated a trace, I would like to sample from the posterior predictive BUT only after fixing z to its mode. i.e. i would like to convert z to an observed variable.

Whats the best way to do this? A few ways that might work are:

  1. Slice the trace down to just the samples where z=my_dersired_values, and enter it into sample_ppc as normal.

  2. Another way would be to build another, almost identical model, with z as observed this time, fit it, then sample_ppc from it.

  3. Use sample_ppc as normal, without z observed, and then remove the posterior predictive samples where z != my_desired_values. This seems inefficient, as a lot of PP samples are not meaningful.

Edit, now that I think on the above options. 1 and 3 are only sensible if the variable z is discrete (which it is), but it would be nice to know what the general solution is when a variable that was not observed, becomes observed. Maybe I would have to specify all “possibly observed” variables in the original model, and just feed missing nan values during inital sampling.

2

I was incorrect in assuming that a pymc3 model could behave as a Bayesian Network, and i could set any paramters/variables and have the model draw samples from any other variables.

After discussing with a college, I think the only way to do this is #2 above (which i implemented, and it worked).
#1 and #3 probaibly would have worked, but are inefficent and the trace object is not simple to work with.

There might be some tricky ways to do this with observed variables and missing values, but given that #2 worked fine, i will ignore that possibility for now.