Sarah is a designer for a business that installs underground sprinkler systems for residential and commercial customers. Sarah designs and presents systems to prospective customers. Brothers Tom and Bill share the ownership responsibilities. Tom is in charge of commercial accounts and Bill is in charge of residential accounts. Sometimes Tom and Bill double book Sarah's time, creating frustration and animosity. Since they share supervisory responsibility, there is no clear way to decide whose accounts are most important. This type of situation represents a violation of Fayol's ____________ principle.