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In statistics, marginal models (Heagerty & Zeger, 2000) are a technique for obtaining regression estimates in multilevel modeling, also called hierarchical linear models. People often want to know the effect of a predictor/explanatory variable X, on a response variable Y. One way to get an estimate for such effects is through regression analysis.
Why the name marginal model?

In a typical multilevel model, there are level 1 & 2 residuals (R and U variables). The two variables form a joint distribution for the response variable ( \) Y_{{ij}} \) ). In a marginal model, we collapse over the level 1 & 2 residuals and thus marginalize (see also conditional probability) the joint distribution into a univariate normal distribution. We then fit the marginal model to data.

For example, for the following hierarchical model,

level 1: $${\displaystyle Y_{ij}=\beta _{0j}+R_{ij}}$$, the residual is $$R_{ij}$$, and $${\displaystyle \operatorname {var} (R_{ij})=\sigma ^{2}}$$

level 2: $${\displaystyle \beta _{0j}=\gamma _{00}+U_{0j}}$$, the residual is $${\displaystyle U_{0j}}$$, and $${\displaystyle \operatorname {var} (U_{0j})=\tau _{0}^{2}}$$

Thus, the marginal model is,

$$} {\displaystyle Y_{ij}\sim N(\gamma _{00},(\tau _{0}^{2}+\sigma ^{2}))}$$

This model is what is used to fit to data in order to get regression estimates.
References
Heagerty, P. J., & Zeger, S. L. (2000). Marginalized multilevel models and likelihood inference. Statistical Science, 15(1), 1-26.

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