Theory of constraints introduces a minimal variable costing system,where only three variables are relevant: throughput,measured as cash received from sales less the cost of materials; operating expenses, which are all organisational expenses other than material costs; and inventory, measured as assets acquired (facilities, equipment and materials) but not yet converted to cash. The goal is to maximise throughput while attempting to keep steady, or preferably reduce, operating expenses and inventory. Under the theory of constraints, direct materials are treated as a variable cost, while direct labour and all other costs are treated as fixed. This minimises the incentives to overproduce, and maximises the incentives to focus on throughput subject to the capacity of the individual production activities of the firm.