# New group level in RStanarm output

**URL:** <https://discourse.mc-stan.org/t/new-group-level-in-rstanarm-output/34366>\
**Category:** rstanarm\
**Created:** [March 6, 2024, 7:38pm UTC](https://discourse.mc-stan.org/t/new-group-level-in-rstanarm-output/34366 "2024-03-06T19:38:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![owen](https://avatars.discourse-cdn.com/v4/letter/o/b2d939/32.png) [@owen](https://discourse.mc-stan.org/u/owen)\
**Post date:** [March 6, 2024, 7:38pm UTC](https://discourse.mc-stan.org/t/new-group-level-in-rstanarm-output/34366/1 "2024-03-06T19:38:24Z")

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I was looking at the `example_model` in rstanarm and a bit unclear about some of the output.

```no-highlight
library(rstanarm)
example(example_model)

tail(example_model$stan_summary[, 1:2])

```

which gives the following output

```no-highlight
#> mean se_mean
#> b[(Intercept) herd:14] 1.02671072 0.016200257
#> b[(Intercept) herd:15] -0.62123581 0.019984998
#> b[(Intercept) herd:_NEW_herd] 0.00726711 0.020258006
#> Sigma[herd:(Intercept),(Intercept)] 0.65040705 0.021683219
#> mean_PPD 1.77508929 0.006152789
#> log-posterior -114.76141580 0.268875928

```

Is `b[(Intercept) herd:_NEW_herd] ` here the mean that each group level intercept is drawn from? Or is it a posterior predictive distribution for a new hypothetical group?

I couldn’t find this documented, but would appreciate a reference!

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**Author:** ![jonah](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/jonah/32/200_2.png) [@jonah](https://discourse.mc-stan.org/u/jonah)\
**Post date:** [March 6, 2024, 7:54pm UTC](https://discourse.mc-stan.org/t/new-group-level-in-rstanarm-output/34366/2 "2024-03-06T19:54:56Z")

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It’s a parameter for a hypothetical new group that is used under the hood when posterior\_predict is called asking for a new group. I didn’t actually realize that it’s included it in the output. I think originally we were intending to keep it hidden and just use it internally as needed.

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<div class="post-metadata">

**Author:** ![owen](https://avatars.discourse-cdn.com/v4/letter/o/b2d939/32.png) [@owen](https://discourse.mc-stan.org/u/owen)\
**Post date:** [March 6, 2024, 7:56pm UTC](https://discourse.mc-stan.org/t/new-group-level-in-rstanarm-output/34366/3 "2024-03-06T19:56:45Z")

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Got it, thanks!