Despite the importance of spelling for both writing and reading, there is considerable disagreement regarding how spelling skills are best acquired. During this and virtually all of the last century, some scholars have argued that spelling should not be directly or formally taught as such instruction is not effective or efficient. We conducted a…

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.…