ABSTRACT

In a wide variety of research areas, analysts are often confronted with hier­ archically structured data. Examples of such data structures include longi­ tudinal data where several observations are nested within individuals, cross­ national data where observations are nested in geographical, political or ad­ ministrative units, data from surveys where respondents are nested under an interviewer, and test data for students within schools (e.g., Longford, 1993). The nested structures give rise to multilevel data and a major problem is to properly analyze the data taking into account the hierarchical structures.