Dear Paul,

Is it true that I cannot provide a categorical variable to the `by`

argument of the `gr()`

function IF the levels of the factor are not numbers?

Kindest regards,

Pascal

- Operating System: CentOS 7
- brms Version: 2.8.0

Dear Paul,

Is it true that I cannot provide a categorical variable to the `by`

argument of the `gr()`

function IF the levels of the factor are not numbers?

Kindest regards,

Pascal

- Operating System: CentOS 7
- brms Version: 2.8.0

I would expect this to work. didn’t it work for you?

It does not work until I recode the factor to numeric values… I will provide the error message later in the day…

ok. a minimal reproducible example would be nice indeed. thanks for taking the time to prepare one!

This example works for me in brms 2.8.0+

```
dat <- data.frame(
y = rnorm(100), x = rnorm(100),
g = rep(1:10, each = 10),
z = factor(rep(c("a", "b", "c", "d", "e"), each = 20))
)
fit <- brm(y ~ x + (1 | gr(g, by = z)), dat)
```

Your example works for me, the following does not work:

```
dat <- data.frame(
y = rnorm(100), x = rnorm(100),
g = rep(1:10, each = 10),
z = factor(rep(c("Control", "Group 1", "Group 2", "Group 3", "Group 4"), each = 20))
)
fit <- brm(y ~ x + (1 | gr(g, by = z)), dat)
```

The error yielded is the following:

```
Compiling the C++ model
Start sampling
Error in FUN(X[[i]], ...) : Stan does not support NA (in Jby_1) in data
failed to preprocess the data; sampling not done
```

Sorry for letting you do all the work and thank you very much!

I think the problem is that your category names have spaces, which

I would recommend to avoid in general. I will see if I can fix this,

but in the meantime, just remove the spaces.

You are absolutely right! I totally neglected the whitespace in the categories!

After replacing the spaces with underscores, everything is working like a charm!

Many thanks!

great! I just fixed it so that whitespaces should now also be handled correctly in the github version of brms.

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