Dear Stan community,

I am using the weight option in the `brm`

function to account for different variances in field sites in a negative binomial generalized linear mixed effect model. Using weights this way is to my understanding approximating the idea of the weighting function `varIdent()`

from the R package `nlme`

(as a reference may serve Galecki & Burzykowski 2013, pages 129-130, paragraph 7.3.2, page 135, paragraph 7.4.3). After considering the Stan code from the `brm`

model, I am still trying to make up my mind, how weights are used for the posteriors. The `brm`

code for weights in R looks as following:

```
b.mod <- brm(y|weights(w) ~ ...)
```

On the one hand, the Stan code from the `brm`

model shows that the posteriors are simply multiplied with weights:

```
vector[N] weights; \\ model weights
target += weights[n] * neg_binommial_2_log_lpmf(Y[n] | mu[n], shape);
```

On the other hand, other R packages seem to implement weights sometimes differently. For example, the `gls()`

function from the `nlme`

package appears to use internally multiplicative inverse of weights, while taking as an input and showing also in the summary output ârawâ weights. Again, the function `lm()`

also differs in the implementation of weights when compared to `gls()`

. So, the way of implementing weights in models can differ from R package to R package, which confused me as to how weights are implemented in the package `brms`

(better to say in Stan as `brm`

relies on Stan).

Does anybody know how `brm()`

uses weights? Does it use internally the multiplicative inverse, which would imply that the user needs to use the ârawâ form of weights (like the weight function in `gls()`

)? Or does the weights are not âtransformedâ, which is why the user should implement the multiplicative inverse a priori coding the model? Many thanks for time and response.

- Operating System: ArchLinux, Linux Kernel 4.16.8-1
- brms Version: 2.3.0