In many experimental evaluations in the social and medical sciences, individuals are randomly assigned to a treatment arm or a control arm of the experiment. After treatment assignment is determined, individuals within one or both experimental arms are frequently grouped together (e.g., within classrooms or schools, through shared case managers,…

Meeting the What Works Clearinghouse (WWC) attrition standard (or one of the attrition standards based on the WWC standard) is now an important consideration for researchers conducting studies that could potentially be reviewed by the WWC (or other evidence reviews). Understanding the basis of this standard is valuable for anyone seeking to meet…