Student achievement as reflected in standardized test scores is not a unitary construct, but a set of interdependent components representing degrees of knowledge of various subject areas. The interdependency of achievement components imply a positive feedback process. Weaknesses in one area reinforce weaknesses in others; strengths in one area…

When the school or district is the unit of analysis in school effects research, the outcome of interest is mean achievement, and the variance explained by school or district level predictors is between schools or between districts. When the individual is the unit of analysis, the outcome of interest is individual achievement and the variance…