To limit the influence of attrition bias in assessments of intervention effectiveness, several federal evidence reviews have established a standard for acceptable levels of sample attrition in randomized controlled trials. These evidence reviews include the What Works Clearinghouse (WWC), the Home Visiting Evidence of Effectiveness Review, and the Teen Pregnancy Prevention Evidence Review. We believe the WWC attrition standard may constitute the first use of model-based, empirically supported bounds on attrition bias in the context of a federally sponsored systematic evidence review. Meeting the 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).

The purpose of this article is to explain the WWC attrition model, how that model is used to establish attrition bounds, and to assess the sensitivity of attrition bounds to key parameter values.

Results are based on equations derived in the article and values generated by applying those equations to a range of parameter values.

The authors find that the attrition boundaries are more sensitive to the maximum level of bias that an evidence review is willing to tolerate than to other parameters in the attrition model.

The authors conclude that the most productive refinements to existing attrition standards may be with respect to the definition of "maximum tolerable bias."