Hi all,

I am planning an individual participant data meta-analysis in brms, in which daily observations are nested in participants, which are further nested in studies - the syntax looks something like this:

```
brm(bf(y ~ 0 + Intercept + covariate + x1 + x2 + x1:x2 +
(1 + x1 | study:pid) + (1 + x1 + x2 + x1:x2 | study),
hu ~ 0 + Intercept + covariate + x1 + x2 + x1:x2 +
(1 + x1 | study:pid) + (1 + x1 + x2 + x1:x2 | study)),
data = metadata, family = hurdle_negbinomial(), prior = metaPriors, sample_prior = TRUE,
iter = 3000, chains = 4, backend = "cmdstanr", threads = threading(7))
```

As you can tell, the dependent variable is a zero-inflated count variable. X1 is a continuous within-participant predictor, X2 is a continuous between-participant predictor. There is missingness both in the within-participant predictor, which is assessed across all studies, and in the between-participant predictor, which is assessed only in a subset of studies (~ 50-60% of studies). My question is, how do I handle this missingness in the model? How do I prevent losing the data from the studies that did not assess x2 in estimating the x1 effect to listwise deletion? My only solution right now is to run a separate model in which I don’t include x2 at all. I don’t think multiple imputation here is feasible due to the complexity of the outcome/model, and this model will already run for multiple days on a supercomputing cluster due to the large amount of data, so imputation would make the computing time explode. I was wondering if there is a way in brms around listwise deletion or possibly to treat missing data as a parameter?

Help would be greatly appreciated!

Best,

Jonas