- K subgroups.
`\(t \in \{S, E\}, Z \in \{0, 1, ..., K-1 \}\)`

- Linear components: $\eta_{t, Z}(\boldsymbol{\theta}) = \xi + \sum_{k=0}^{K-1}\left{\beta_{k} + \tau_{k} I(t=E)\right} I(Z=k)$
- reference group:
`\(\beta\_0 = 0\)`

`\(\theta = (\xi, \beta, \tau)\)`