Meridian supports national-level models, although we recommend that you
use the geo-level model when possible. The national-level model is a special
case of the geo-level model with only a single geo. There is no separate model
class.

The following parameter restrictions apply, and are automatically enforced, when
you use the national-level model:

- \\(\\eta\^{\[M\]}_i=0\\) and \\(\\beta_{1,i}\^{\[M\]}=\\beta_i\^{\[M\]} \\ \\ \\forall i\\)
- \\(\\eta\^{\[OM\]}_i=0\\) and \\(\\beta_{1,i}\^{\[OM\]}=\\beta_i\^{\[OM\]} \\ \\ \\forall i\\)
- \\(\\eta\^{\[RF\]}_i=0 \\) and \\(\\beta\^{\[RF\]}_{g,i}=\\beta\^{\[RF\]}_i \\ \\ \\forall i\\)
- \\(\\eta\^{\[ORF\]}_i=0 \\) and \\(\\beta\^{\[ORF\]}_{g,i}=\\beta\^{\[ORF\]}_i \\ \\ \\forall i\\)
- \\(\\xi_i\^{\[C\]}=0\\) and \\(\\gamma_{1,i}\^{\[C\]}=\\gamma_i\^{\[C\]} \\ \\ \\forall c\\)
- `unique_sigma_for_each_geo = False`

> [!NOTE]
> **Note:** Population scaling has no effect in the national-level model.