# Posterior predictive sampling with shared matrix
**URL:** https://discourse.pymc.io/t/posterior-predictive-sampling-with-shared-matrix/1848
**Category:** Questions
**Created:** [August 31, 2018, 8:57am UTC](https://discourse.pymc.io/t/posterior-predictive-sampling-with-shared-matrix/1848 "2018-08-31T08:57:06Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@dlepmet](https://discourse.pymc.io/u/dlepmet)
#### Post date: [August 31, 2018, 8:57am UTC](https://discourse.pymc.io/t/posterior-predictive-sampling-with-shared-matrix/1848/1 "2018-08-31T08:57:06Z")
Hello,
I was trying to predict values for unseen data using a regression model, e.g. Y = \alpha + X \beta with Y, X and \beta being matrices of sizes (n,1), (n,m) and (m,1) respectively. I tried to use a shared variable for X. The model ran fine, however, changing X and using the sample\_ppc function to predict values always led to an error. I then tried to reproduce the error with simpler models. The following model which is based on an example from [https://docs.pymc.io/advanced\_theano.html](https://docs.pymc.io/advanced_theano.html) yields the same error.
```
x = np.random.randn(100)
y = x > 0
x_shared = theano.shared(x.reshape(-1,1))
with pm.Model() as model:
coeff = pm.Normal('x', mu=0, sd=1)
logistic = pm.math.sigmoid(coeff * x_shared)
pm.Bernoulli('obs', p=logistic, observed=y.reshape(-1,1))
# fit the model
trace = pm.sample()
# Switch out the observations and use `sample_posterior_predictive` to predict
x_new = np.array([-1, 0, 1.]).reshape(-1,1)
x_shared.set_value(x_new)
post_pred = pm.sample_ppc(trace, samples=500)
```
The error from the sample\_ppc is
```
TypeError: Attempted to generate values with incompatible shapes:
size: 1
dist_shape: (3,)
broadcast_shape: (3, 1)
```
If I turn both the shared variable and the observed data into vectors by removing the reshape(-1,1) everything works. Am I missing something or is this not possible due to limited support of changing shapes?
Thanks!
---
### Author:  [@junpenglao](https://discourse.pymc.io/u/junpenglao)
#### Post date: [August 31, 2018, 9:26am UTC](https://discourse.pymc.io/t/posterior-predictive-sampling-with-shared-matrix/1848/2 "2018-08-31T09:26:49Z")
Seems to be a bug - could you open an issue on Github?
---
### Author:  [@dlepmet](https://discourse.pymc.io/u/dlepmet)
#### Post date: [August 31, 2018, 11:07am UTC](https://discourse.pymc.io/t/posterior-predictive-sampling-with-shared-matrix/1848/3 "2018-08-31T11:07:18Z")
Issue is now opened: [#3180](https://github.com/pymc-devs/pymc3/issues/3180)