Many assignment problems require the solution to be balanced.Such a problem is the Balanced Academic Curriculum Problem (BACP) [1]. Standard deviation is a common way to measure the balance of a set of values.
Schaus, P., Deville, Y., Dupont, P., & Régin, J.-C. (2006). Simplification and extension of the SPREAD Constraint. Third International Workshop on Constraint Propagation And Implementation (CPAI′06), Nantes, France. https://hdl.handle.net/2078.5/254089