# Prior sample from TruncatedNormal gives inf
**URL:** https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316
**Category:** Questions
**Created:** [May 22, 2019, 3:36pm UTC](https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316 "2019-05-22T15:36:25Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@rpgoldman](https://discourse.pymc.io/u/rpgoldman)
#### Post date: [May 22, 2019, 3:36pm UTC](https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316/1 "2019-05-22T15:36:25Z")
I have made a model with the following observed value, `obs`:
```auto
pm.TruncatedNormal("obs", mu=multiplier.T * pred + means_vector, sd=err_sd, observed=self.df['mean_log_gfp_live'].to_numpy(),
lower=0.0, upper=12.0)
```
I then sampled from the model using `sample _prior_predictive()`.
I was quite surprised that many of the sampled values are apparently `np.inf` or `-np.inf`. Shouldn’t this be impossible because of the truncation? Or at best, shouldn’t this give a runtime error?
When I checked, 1977 of my 5000 samples have at least one of their values (`observed` is a vector of 1043 elements) either `inf` or `-inf`.
Probably this means there’s something wrong with my model, but this also suggests that this variable can behave oddly in prior sampling.
---
### Author:  [@rpgoldman](https://discourse.pymc.io/u/rpgoldman)
#### Post date: [May 22, 2019, 3:50pm UTC](https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316/2 "2019-05-22T15:50:48Z")
The `sd` variable is a `HalfNormal` – is there any chance that this is going awry when I get too high a sample there, and we hit this code in the `TruncatedNormal` sampler:
```auto
return generate_samples(stats.truncnorm.rvs,
a=(a_v - mu_v)/std_v,
b=(b_v - mu_v) / std_v,
loc=mu_v,
scale=std_v,
dist_shape=self.shape,
size=size,
)
```
If that’s the case, should we do something to trap this, even if it’s only trapping infinite outputs from the various `rvs` methods?
And is there something I should be doing to limit my `err_sd` to keep this from happening?
---
### Author:  [@junpenglao](https://discourse.pymc.io/u/junpenglao)
#### Post date: [May 25, 2019, 6:01am UTC](https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316/3 "2019-05-25T06:01:38Z")
hmmm this sounds like a bug.
---
### Author:  [@rpgoldman](https://discourse.pymc.io/u/rpgoldman)
#### Post date: [May 27, 2019, 4:53pm UTC](https://discourse.pymc.io/t/prior-sample-from-truncatednormal-gives-inf/3316/4 "2019-05-27T16:53:16Z")
Thanks for letting me know. I will try to isolate a test case.