**Cluster:**A grouping containing*lower level*elements. For example in a sample survey the set of households in a neighbourhood.**Cross classification**: A structure where lower level units are grouped within the cells of a multiway classification of higher level units**Design matrix:**In the fixed part of the model, the matrix of values of the explanatory variables*X*. In the random part the matrix of explanatory variables*Z*.**Explanatory variable**: Also known as an ‘independent’ variable. In the fixedpart of the model usually denoted by*X*and in the random part by*Z*.**Fixed part:**That part of a model represented by*X*ß, that is the average relationship.**Level**:A component of a data hierarchy. Level 1 is the lowest level, for example students within schools or repeated measurement occasions within individual subjects.**Level**: The variation among level*n*variation*n*unit measurements.**Multiple membership**: A structure where a level unit may be nested within one or more higher level units.**Nesting**:The clustering of units into a hierarchy**Random part**:That part of a model represented by*Zu*, that is the contribution of the random variables, at each level.**Response variable:**Also known as a ‘dependent’ variable. Denoted by*y*.**Unit**:An entity defined at a level of a data hierarchy. For example an individual student will be a level 1 unit within a level 2 unit such as a school.

The above terms are courtesy of: Goldstein, H. (2003). *
Multilevel Statistical Models* (3rd Edition). London, Edward Arnold: New York, Halstead Press.

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