# Trying to understand default prior for the hurdle part

**URL:** <https://discourse.mc-stan.org/t/trying-to-understand-default-prior-for-the-hurdle-part/24337>\
**Category:** brms\
**Tags:** priors\
**Created:** [September 10, 2021, 8:38am UTC](https://discourse.mc-stan.org/t/trying-to-understand-default-prior-for-the-hurdle-part/24337 "2021-09-10T08:38:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![ayw](https://avatars.discourse-cdn.com/v4/letter/a/3da27b/32.png) [@ayw](https://discourse.mc-stan.org/u/ayw)\
**Post date:** [September 10, 2021, 8:38am UTC](https://discourse.mc-stan.org/t/trying-to-understand-default-prior-for-the-hurdle-part/24337/1 "2021-09-10T08:38:31Z")

</div>

Hi my name is Andy. I am new to brms and Bayesian analysis in general.  
I’m trying to understand hurdle models and playing around with understanding effects of priors on hurdle\_lognormal with prior predictive checks.  
My question is more basic. I noticed with prior\_summary(), the default prior for Intercept\_hu is “logistic(0, 1)”, rather than a distribution. What does that mean? The only other discussion I could find was here: [Choosing priors for a brms hurdle model](https://discourse.mc-stan.org/t/choosing-priors-for-a-brms-hurdle-model/16445) where jobi asked if putting a prior as “logistic(3, 1)” was appropriate. What does “logistic(3, 1)” as a prior mean? Would a normal(0, 1.5) be appropriate as a prior for the intercept in hurdle models?  
I couldn’t find much about priors for hurdle models on google or stack exchange either, so am wondering if anyone can point me to the right direction.  
Thank you very much.

---

<div class="post-metadata">

**Author:** ![FJCC](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/fjcc/32/6653_2.png) [@FJCC](https://discourse.mc-stan.org/u/FJCC)\
**Post date:** [September 11, 2021, 3:41am UTC](https://discourse.mc-stan.org/t/trying-to-understand-default-prior-for-the-hurdle-part/24337/2 "2021-09-11T03:41:57Z")

</div>

I can’t help you with hurdle models but I will point out that there is a logistic distribution. It looks roughly like a normal distribution but with fatter tails. The logistic distribution is parameterized with a location \mu and a scale s. I assume, not having tried to find the function definition, that logistic(0,1) has \mu = 0 and s = 1. I plotted logistic(0,1), using the dlogis function in R, and normal(0,1.5) and they are fairly similar, though the logistic distribution does have a little more amplitude beyond roughly |2.5|.

---

<div class="post-metadata">

**Author:** ![ayw](https://avatars.discourse-cdn.com/v4/letter/a/3da27b/32.png) [@ayw](https://discourse.mc-stan.org/u/ayw)\
**Post date:** [September 11, 2021, 7:09am UTC](https://discourse.mc-stan.org/t/trying-to-understand-default-prior-for-the-hurdle-part/24337/3 "2021-09-11T07:09:00Z")

</div>

Thank you so much! I knew it was something basic and relating to my lack of knowledge! Thanks again!