The extent to which an evaluation is truthful or valid in the scope and detail of what it communicates about a context, program, project, or any of their components.
Yarbrough, D. B., Shulha, L. M., Hopson, R. K., and Caruthers, F. A. (2011). The program evaluation standards: A guide for evaluators and evaluation users (3rd ed.). Thousand Oaks, CA: Sage.