# Mixed model

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Mixed model

A mixed model is a statistical model containing both fixed effects and random effects, that is mixed effects. These models are useful in a wide variety of disciplines in the physical, biological and social sciences. They are particularly useful in settings where repeated measurements are made on the same statistical units, or where measurements are made on clusters of related statistical units.

## History and current status

Ronald Fisher introduced random effects models to study the correlations of trait values between relatives. In the 1950s, Charles Roy Henderson provided best linear unbiased estimates (BLUE) of fixed effects and best linear unbiased predictions (BLUP) of random effects. Subsequently, mixed modeling has become a major area of statistical research, including work on computation of maximum likelihood estimates, non-linear mixed effect models, missing data in mixed effects models, and Bayesian estimation of mixed effects models. Mixed models are applied in many disciplines where multiple correlated measurements are made on each unit of interest. They are prominently used in research involving human and animal subjects in fields ranging from genetics to marketing, and have also been used in industrial statistics.[citation needed]

## Definition

In matrix notation a mixed model can be represented as $\ y = X \beta + Zu + \epsilon\,\!$

where

• y is a vector of observations, with mean E(y) = Xβ
• β is a vector of fixed effects
• u is a vector of independent and identically-distributed (IID) random effects with mean E(u) = 0 and variance-covariance matrix $\operatorname{var}(u)=G$
• $\epsilon$ is a vector of IID random error terms with mean $E(\epsilon)=0$ and variance $\operatorname{var}(\epsilon)=R$
• X and Z are matrices of regressors relating the observations y to β and u

## Estimation

Henderson's "mixed model equations" (MME) are:[citation needed] $\begin{pmatrix} X'R^{-1}X & X'R^{-1}Z \\ Z'R^{-1}X & Z'R^{-1}Z + G^{-1} \end{pmatrix}\begin{pmatrix} \tilde{\beta} \\ \tilde{u} \end{pmatrix}=\begin{pmatrix} X'R^{-1}y \\ Z'R^{-1}y \end{pmatrix}$

The solutions to the MME, $\textstyle\tilde{\beta}$ and $\textstyle\tilde{u}$ are best linear unbiased estimates (BLUE) and predictors for β and u, respectively. This is a consequence of the Gauss-Markov theorem when the conditional variance of the outcome is not scalable to the identity matrix. When the conditional variance is known, then the inverse variance weighted least squares estimate is BLUE. However, the conditional variance is rarely, if ever, known. So it is desirable to jointly estimate the variance and weighted parameter estimates when solving MMEs.

One method used to fit such mixed models is that of the EM algorithm where the variance components are treated as unobserved nuisance parameters in the joint likelihood. Currently, this is the implemented method for the major statistical software packages R (lme in the nlme library) and SAS (proc mixed). The solution to the mixed model equations is a maximum likelihood estimate when the distribution of the errors is normal. 

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