The cost system for inventory valuation may have been developed to provide a reasonable total cost of inventory and a reasonable total cost of goods sold in order to have reasonably accurate financial statements. If a company has small inventory amounts and significant sales, a simple cost system that spreads manufacturing overhead costs solely on the basis of machine hours can result in a reasonably accurate balance sheet and income statement.
While a simple cost system using just one cost driver (machine hours) may result in accurate financial statements, it often fails to provide the true cost of individual products that vary in complexity. For example, one product might require very few machine hours but will require many hours of special handling. The costs assigned on the basis of machine hours alone will be too low in relationship to the true cost of manufacturing this product. Another product might require many machine hours but no other activities. This product's cost will be overstated because the rate assigned via the machine hours will include an amount for other activities that generally occur for the other products manufactured.
A cost system developed for inventory valuation is limited to the cost of direct materials, direct labor, and manufacturing overhead. The total cost of providing products to a customer will also include nonmanufacturing expenses. One customer might require a company to incur additional selling, delivering, storing, and administrative expenses. Another customer might not require any of those activities and their related expenses.
Activity based costing attempts to calculate the true cost of a product and customer by assigning costs and expenses based on their root causes. Because there are many root causes, the company will assign costs based on many cost drivers. This results in more accuracy for the cost and expense of a specific product for a specific customer than simply spreading the manufacturing costs on the basis of one cost driver such as machine hours.