METHODOLOGICAL ISSUES IN EDUCATIONAL PERFORMANCE AND SCHOOL EFFECTIVENESS RESEARCH - A DISCUSSION WITH WORKED EXAMPLES

There has been a growing awareness among educational researchers of th e consequences of using data-analytic models that fail to account for the inherent clustered or hierarchical sampling structure of the data typically obtained. Such clustering poses special analytic problems re lated to levels of analysis, aggregation bias, heterogeneity of regres sion and parameter mis-estimation, with important implications for the correct interpretation of effects. This paper compares the results ob tained from fitting single-level and multilevel models to two hierarch ically structured data sets designed to explain variation in student a chievement. Emphasis is given to the crucial importance of fitting mod els commensurate with the sampling structure of the data to which they are applied.