Experimental evaluations that involve the educational system usually involve a hierarchical structure (students are nested within classrooms that are nested within schools, etc.). Concerns about contamination, where research subjects receive certain features of an intervention intended for subjects in a different experimental group, have often led…

Explains the characteristics and importance of random assignment in experimental research on education. Considers factors supporting causal inferences, the nature of "assignment units," differences between random sampling and random assignment, the characteristics of randomization tests, forms of random assignment, and effects of experimental…

A number of potential threats to internal validity in true experiments are discussed. Programmatic research efforts proceeding from established correlational findings to experiments employing random assignment are described and used as referents in discussions of validity considerations. The collection of observational data is recommended for use…

Problems with conducting randomized field experiments in education are explored. Focus is on problems encountered while evaluating a group of dropout prevention projects. Project planners were asked to manipulate the subject eligibility criteria until they identified as eligible three to four times as many students as they could serve. They were…