# Sample\_ppc fails on traces containing a subset of variables

**URL:** <https://discourse.pymc.io/t/sample-ppc-fails-on-traces-containing-a-subset-of-variables/323>\
**Category:** Questions\
**Tags:** from\_github\
**Created:** [September 13, 2017, 7:00am UTC](https://discourse.pymc.io/t/sample-ppc-fails-on-traces-containing-a-subset-of-variables/323 "2017-09-13T07:00:30Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![junpenglao](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/junpenglao/32/8_2.png) [@junpenglao](https://discourse.pymc.io/u/junpenglao)\
**Post date:** [September 13, 2017, 7:00am UTC](https://discourse.pymc.io/t/sample-ppc-fails-on-traces-containing-a-subset-of-variables/323/1 "2017-09-13T07:00:30Z")

</div>

[Github issue #2561](https://github.com/pymc-devs/pymc3/issues/2561)

Trying to use `sample_ppc` using a trace that does not contain all of the model’s RVs fails with “theano.gof.fg.MissingInputError: Undeclared input”.

Basically, I want to sample the posterior predictive distribution for a new dataset using a hierarchical model that I have sampled from on training dataset. So the top-level and group-level variables are present in the trace, but the latent variables for the individual units are not.

I am open to the argument that this is a misuse of `sample_ppc`, since this isn’t really a check so much as an attempt at inference. The functionality for doing this is essentially the same as in `sample_ppc` though; just drawing random values in depth-first order seems to fix the problem.

If this seems likely to be of sufficiently general interest, I’d be happy to send a pull request for the fix that I have in mind.

Failing example:

```python
with pm.Model() as model:
    a = pm.Gamma('a', mu=10.0, sd=2.0)
    b = pm.Gamma('b', mu=a, sd=2.0)

    trace = pm.sample(trace=[model.a, model.a_log__])
    assert len(trace.varnames) == 2

    c = pm.Gamma('c', mu=b, sd=1.0)
    d = pm.Normal('d', mu=c, sd=a)

    ppc = pm.sample_ppc(trace, 100, vars=[c,d]) #!!! will throw theano.gof.fg.MissingInputError

```