Ignoring a level can have a substantial impact on the conclusions of a multilevel analysis. For intercept-only models and for balanced data, we derive these effects analytically. For more complex random intercept models or for unbalanced data, a simulation study is performed. Most important effects concern estimates and corresponding standard…

Introduction to the two articles in this issue. Provides background information on a longitudinal study of Flanders secondary schools and on the study's sample, method, and variables relevant to the two articles. Also describes the organization of secondary education in Flanders. (Contains 33 references.) (PKP)

Explores the effects of secondary schools, teachers, and classes on mathematics achievement in Flanders secondary schools. After controlling for within-class and school recruitment differences, finds that class-level group composition remains an important explanation of difference in mathematics achievement. Also finds that the school-level…

Reports on the effects of individual background characteristics, classes, and school on four noncognitive secondary education outcomes in Flanders: Degree to which the student feels at home in the school environment, extent to which the student does his/her best for the schoolwork, academic self-concept, and social integration in the class.…

This study addresses the effects of secondary schools and classes on language achievement in Flanders, Belgium. The results of a three-level analysis (students within classes within schools) indicate that the group composition at the class level is very important. In classes with a high average initial cognitive ability or a large proportion of…