A statistical measure of the degree of relationship between or among variables. It is expressed in the form of an index that may vary from -1.00 to +1.00, with 0 representing the absence of co-variation, and +1 and -1 represent per co-variation, albeit in different directions.
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.