# Need help understanding hurdle (hurdle\_gamma) models using brms

**URL:** <https://discourse.mc-stan.org/t/need-help-understanding-hurdle-hurdle-gamma-models-using-brms/16736>\
**Category:** brms\
**Created:** [July 18, 2020, 7:39pm UTC](https://discourse.mc-stan.org/t/need-help-understanding-hurdle-hurdle-gamma-models-using-brms/16736 "2020-07-18T19:39:54Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Srivatsa\_Srinath](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/srivatsa_srinath/32/5754_2.png) [@Srivatsa\_Srinath](https://discourse.mc-stan.org/u/Srivatsa_Srinath)\
**Post date:** [July 18, 2020, 7:39pm UTC](https://discourse.mc-stan.org/t/need-help-understanding-hurdle-hurdle-gamma-models-using-brms/16736/1 "2020-07-18T19:39:54Z")

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Hi,

I am trying to use hurdle gamma model for one of my use cases, to handle a zero-inflated scenario. I have a very simple code creating dummy data with quite a few zeros.

```
# Dataset prep

non_zero <- rbinom(1000, 1, 0.1)
g_vals <- rgamma(n = 1000, shape = 2, scale = 2)
dat <- data.frame(x = non_zero * g_vals)

```

The model is written as

```
hum <- brm(bf(x ~ 1, hu ~ 1), data = dat, family = hurdle_gamma)

```

I would like to understand the results and the associated parameters.

A plot of the predicted results from the model using

```
tibble(x=1) %>% add_fitted_draws(hum) %>% ggplot(aes(x = .value)) + geom_density()

```

is as follows

 ![image](https://canada1.discourse-cdn.com/flex030/uploads/mc_stan/original/2X/d/d510c7cc350c6feb652b87338696438a7db93bed.png)

The posterior summary is:

```
                  Estimate Est.Error Q2.5 Q97.5
b_Intercept 1.468677 0.1037202 1.271352 1.681500
b_hu_Intercept 2.081498 0.1474433 1.802057 2.372279
shape 1.757053 0.3114681 1.203522 2.418776
lp__ -315.947968 1.2657819 -319.090956 -314.508678

```

I don’t see any divergences in the model fit. Also there is pretty good mixing of the chains for the parameters. Given the whole reason for me to consider the hurdle model was to see a model predict zeros in abundance, I am unable to understand the predictions. Shouldn’t I see a lot of zeros?

It would be great if someone can throw light on the share a simple test case on using the hurdle model. I am unable to find a nice write up of modeling using hurdle\_gamma using brms.

- Operating System: Ubuntu 18.04
- brms Version:2.13

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<div class="post-metadata">

**Author:** ![franzsf](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/franzsf/32/8545_2.png) [@franzsf](https://discourse.mc-stan.org/u/franzsf)\
**Post date:** [July 18, 2020, 9:32pm UTC](https://discourse.mc-stan.org/t/need-help-understanding-hurdle-hurdle-gamma-models-using-brms/16736/2 "2020-07-18T21:32:32Z")

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Try using the predict functions in brms, or analogs. It looks like you’re estimating fitted results, which are conditional averages, not draws from the posterior predictive distribution.

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<div class="post-metadata">

**Author:** ![Srivatsa\_Srinath](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/srivatsa_srinath/32/5754_2.png) [@Srivatsa\_Srinath](https://discourse.mc-stan.org/u/Srivatsa_Srinath)\
**Post date:** [July 19, 2020, 2:49am UTC](https://discourse.mc-stan.org/t/need-help-understanding-hurdle-hurdle-gamma-models-using-brms/16736/3 "2020-07-19T02:49:21Z")

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Ah! Thanks for the suggestion. I used add\_predicted\_draws instead and it does seem to give me results as I was looking for. I was incorrectly using add\_fitted\_draws instead.