A value expressing the degree to which some characteristic or relation is to be found in specified instances; e.g., the coefficient of correlation is a value expressing the degree to which two variables vary concomitantly.
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.