Variabilities in production and distribution systems are those unexpected occurrences that upset or alter pre-planned production activities. As a way of overcoming such variabilities, JIT systems use existing inventories judiciously and tactically. For example, “the 'extra' inventory is then used to cover variations or problems. Effective inventory tactics require 'just in time', not 'just in case'. JIT inventory is the minimum inventory necessary to keep a perfect system running. With JIT inventory, the exact amount of goods arrives at the moment it is needed, not a minute before or a minute after.” (Chapter 16, p.495) One way in which JIT can help remove variability is by “eliminating inventory that hides variability in the production system”. A suitable analogy to illustrate this point is a lake full of rocks. “The water in the lake represents inventory flow, and the rocks represent problems such as late deliveries, machine breakdowns, and poor personnel performance. The water level in the lake hides variability and problems. Because inventory hides problems, they are hard to find”. (Chapter 16, p.495) JIT can help reduce variability here by reducing unwanted inventory. The top management can greatly reduce variability by adopting JIT practices.