For more accurate product costing, pioneering accountants use activity-based costing that backtracks as many costs as possible to products based on the activities which have caused the costs to occur (for example Cooper and Kaplan 1988). If some costs cannot be traced directly to activities, activity-based costing still allocates these costs using the most logical basis first to departments and then to products; that is, activity-based costing converts 'fixed' costs into 'variable' costs. For example, total power cost can be allocated first to each department on the basis of kilowatt hours, and then the departmental power cost is allocated to each product based on machine hours used to make the product in the department.