Background: Deficient vocabulary is a frequently reported symptom of developmental language impairment, but the nature of the deficit and its developmental course are not well documented. Aims: To describe the nature of the deficit in terms of breadth and depth of vocabulary knowledge and to determine whether the nature and the extent of the…

Background: Cluster-randomized experiments that assign intact groups such as schools or school districts to treatment conditions are increasingly common in educational research. Such experiments are inherently multilevel designs whose sensitivity (statistical power and precision of estimates) depends on the variance decomposition across levels.…