The issue of whether the unit of assignment is also used as the unit of analysis is an extremely important one. As Biglan and Ary (1985) and Kirby, et al. (1995) have said, when units of assignment and analysis are mixed, school or classroom differences may be confounded with program effects on individuals. However, this issue is not as straightforward in programs that last for multiple years in which participants may change classrooms or schools, communities, or other organizational structures to which they were originally assigned.
Nevertheless, almost half of the effective positive youth development interventions matched unit of analysis and unit of assignment. By far the most common situation involved individual units of assignment and analysis. On the other hand, in very large studies in which classrooms, schools, or even school districts were the unit of assignment, the investigators often chose to use individual subject scores as the unit of analysis. Ideally programs should address the problem by using multi-level analytical techniques such as hierarchical linear modeling.