While previous research indicates that a variety of competencies may distinguish effective principals from less effective principals, these vary and are numerous. For instance, Cotton (2003) identified 25 categories of principal behaviors and aptitudes that were not correlated to student achievement but indirectly, positively affected student behaviors, teacher behaviors, and dropout rates. Marzano et al. (2005) described 21 leadership practices or behaviors that were positively correlated to student achievement.