# Categorical interaction effects while using index notation in {brms} models

**URL:** <https://discourse.mc-stan.org/t/categorical-interaction-effects-while-using-index-notation-in-brms-models/34091>\
**Category:** brms\
**Tags:** interpret-results\
**Created:** [February 12, 2024, 11:13pm UTC](https://discourse.mc-stan.org/t/categorical-interaction-effects-while-using-index-notation-in-brms-models/34091 "2024-02-12T23:13:44Z")\
**Posts on this page:** 1\
**Showing post:** 4

<div class="post-metadata">

**Author:** ![aobrien](https://avatars.discourse-cdn.com/v4/letter/a/f19dbf/32.png) [@aobrien](https://discourse.mc-stan.org/u/aobrien)\
**Post date:** [February 14, 2024, 1:34am UTC](https://discourse.mc-stan.org/t/categorical-interaction-effects-while-using-index-notation-in-brms-models/34091/4 "2024-02-14T01:34:43Z")

</div>

Although it doesn’t answer the interpretation question between **Model 1** and **Model 2** , I did stumble across this thread by @Solomon, which provides input into how this interaction could be specified: [How to properly compare interacting levels - #6 by Solomon](https://discourse.mc-stan.org/t/how-to-properly-compare-interacting-levels/20457/6)

For my example this would be outlined as below; it appears to be equivalent to **Model 1**.

```r
outcome ~ 0 + a + b + c,
         a ~ 0 + group,
         b ~ 0 + covar,
         c ~ (0 + group) : (0 + covar)

```

---

_[View the full topic](https://discourse.mc-stan.org/t/categorical-interaction-effects-while-using-index-notation-in-brms-models/34091)._