## 6.5 Mixed models

A mixed model is a regression model of observations that allows for random variation at two different levels. In this section we will focus on mixed models in an exponential family context. Mixed models can be considered in greater generality but there will then be little shared structure and one has to deal with the models much more on a case-by-case manner.

In a mixed model we have observations $$y_j \in \mathcal{Y}$$ and $$z \in \mathcal{Z}$$ such that:

• the distribution of $$z$$ is an exponential family with canonical parameter $$\theta_0$$
• conditionally on $$z$$ the $$y_j$$s are independent with a distribution from an exponential family with sufficient statistics $$t_j( \cdot \mid z)$$.

This definition emphasizes how the $$y_j$$s have variation at two levels. There is variation in the underlying $$z$$, which is the first level of variation (often called the random effect), and then there is variation among the $$y_j$$s given the $$z$$, which is the second level of variation. It is a special case of hierarchical models (Bayesian networks with tree graphs) also known as multilevel models, with the mixed model having only two levels. When we observe data from such a model we typically observe independent replications, $$(y_{ij})_{j=1, \ldots, m_i}$$ for $$i = 1, \ldots, n$$, of the $$y_j$$s only. Note that we allow for a different number, $$m_i$$, of $$y_j$$s for each $$i$$.

The simplest class of mixed models is obtained by $$t_j = t$$ not depending on $$j$$, and $t(y_j \mid z) = \left(\begin{array}{cc} t_1(y_j) \\ t_2(y_j, z) \end{array} \right)$ for some fixed maps $$t_1 : \mathcal{Y} \mapsto \mathbb{R}^{p_1}$$ and $$t_2 : \mathcal{Y} \times \mathcal{Z} \mapsto \mathbb{R}^{p_2}$$. This is called a random effects model (this is a model where there are no fixed effects in the sense that $$t_j$$ does not depend on $$j$$, and given the random effect $$z$$ the $$y_j$$s are i.i.d.). The canonical parameters associated with such a model are $$\theta_0$$ that enters into the distribution of the random effect, $$\theta_1 \in \mathbb{R}^{p_1}$$ and $$\theta_2 \in \mathbb{R}^{p_2}$$ that enter into the conditional distribution of $$y_j$$ given $$z$$.

Example 6.4 The special case of a Gaussian, linear random effects model is the model where $$z$$ is $$\mathcal{N}(0, 1)$$ distributed, $$\mathcal{Y} = \mathbb{R}$$ (with base measure proportional to Lebesgue measure) and the sufficient statistic is $t(y_j \mid z) = \left(\begin{array}{cc} y_j \\ - y_j^2 \\ zy_j \end{array}\right).$ There are no free parameters in the distribution of $$z$$.

From Example 6.2 it follows that the conditional variance of $$y$$ given $$z$$ is $\sigma^2 = \frac{1}{2\theta_2}$ and the conditional mean of $$y$$ given $$z$$ is $\frac{\theta_1 + \theta_3 z}{2 \theta_2} = \sigma^2 \theta_1 + \sigma^2 \theta_3 z.$ Reparametrizing in terms of $$\sigma^2$$, $$\beta_0 = \sigma^2 \theta_1$$ and $$\nu = \sigma^2 \theta_3$$ we see how the conditional distribution of $$y_j$$ given $$z$$ is $$\mathcal{N}(\beta_0 + \nu z, \sigma^2)$$. From this it is clear how the mixed model of $$y_j$$ conditionally on $$z$$ is a regression model. However, we do not observe $$z$$ in practice.

Using the above distributional result we can see that the Gaussian random effects model of observations $$y_{ij}$$ can equivalently be stated as

$Y_{ij} = \beta_0 + \nu Z_i + \varepsilon_{ij}$

for $$i = 1, \ldots, n$$ and $$j = 1, \ldots, m_i$$ where $$Z_1, \ldots, Z_n$$ are i.i.d. $$\mathcal{N}(0, 1)$$-distributed and independent of $$\varepsilon_{11}, \varepsilon_{12}, \ldots, \varepsilon_{1n_1}, \ldots, \varepsilon_{mn_m}$$ that are themselves i.i.d. $$\mathcal{N}(0, \sigma^2)$$-distributed.