Cluster Size and Aggregated Level 2 Variables in Multilevel Models. A Cautionary Note

Abstract

This paper explores the consequences of small cluster size for parameter estimation in multilevel models. In particular, the interest lies in  parameter estimates (regression weights) in linear multilevel models of level 2 variables that are functions of level 1 variables, as for instance the cluster-mean of a certain property, e.g. the average income or the proportion of certain people in a neighborhood. To this end, a simulation study is used to determine the
effect of varying cluster sizes and number of clusters. The results show that small cluster sizes can cause severe downward bias in estimated regression weights of aggregated level 2 variables. Bias does not decrease if the number of clusters (i.e. the level 2 units) increases.