Hello, I am thinking about using the synthetic control method to evaluate an intervention that took place across several geographic units (so I have multiple treatment units). I saw on the synth command documentation that only a single treatment unit may be specified but if there are multiple units, users can think about combining them.

I'm curious how combining units works in practice. For example, do people take an average or weighted average of their predictors and outcomes across all the treatment units (though how would this work for categorical variables)? Are there other methods for doing this?

Thanks!