# Unexpected results

**URL:** <https://discourse.pymc.io/t/unexpected-results/2733>\
**Category:** Questions\
**Created:** [February 14, 2019, 9:06pm UTC](https://discourse.pymc.io/t/unexpected-results/2733 "2019-02-14T21:06:16Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![madarshahian](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/madarshahian/32/722_2.png) [@madarshahian](https://discourse.pymc.io/u/madarshahian)\
**Post date:** [February 14, 2019, 9:06pm UTC](https://discourse.pymc.io/t/unexpected-results/2733/1 "2019-02-14T21:06:16Z")

</div>

Is it wrong to expect this very simple model gives me a joint distribution like a line for the posterior? What I get is actually my prior.

```
import numpy as np
import pymc3 as pm
import theano
import matplotlib.pyplot as plt
#%%
y=4
y_er=.1
y_observed=y_er*np.random.randn(5)+y
#%%
with pm.Model() as m:
    a = pm.Uniform('a', 0, 5)
    b = pm.Uniform('b', 0, 5)
    yhat = a+b
    sigma = pm.HalfCauchy('sd', .1)
    step=pm.NUTS()
    obs = pm.Normal('obs', yhat, sigma,observed=y_observed)
#%%
trace = pm.sample(model=m,draws=2000,tune=1000,skip=500,njobs=1,step=step)
a_pos=trace['a']
b_pos=trace['b']
plt.figure()
plt.scatter(a_pos,b_pos,color='red',alpha=.1)
plt.xlabel('a')
plt.ylabel('b')

```

![Figure_4](https://canada1.discourse-cdn.com/flex036/uploads/pymc3/original/2X/f/fe6d4b23cbc464ab1e038f7b2a5292995e17ca89.png)

---

<div class="post-metadata">

**Author:** ![aloctavodia](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/aloctavodia/32/8642_2.png) [@aloctavodia](https://discourse.pymc.io/u/aloctavodia)\
**Post date:** [February 14, 2019, 11:50pm UTC](https://discourse.pymc.io/t/unexpected-results/2733/2 "2019-02-14T23:50:27Z")

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It seems to me you are sampling from the prior, because you are defining the step before defining the likelihood, if you write:

```auto
    obs = pm.Normal('obs', yhat, sigma,observed=y_observed)
    step=pm.NUTS()

```

you will get the correct result.

BTW, you can just let PyMC3 choose the step for you by just writing.

```auto
    trace = pm.sample(2000)

```

---

<div class="post-metadata">

**Author:** ![madarshahian](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/madarshahian/32/722_2.png) [@madarshahian](https://discourse.pymc.io/u/madarshahian)\
**Post date:** [February 15, 2019, 3:53pm UTC](https://discourse.pymc.io/t/unexpected-results/2733/3 "2019-02-15T15:53:54Z")

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Good catch! What a typo! Thank you Osvaldo!