Ideally, a cost driver is an activity that is the root cause of why a cost occurs.
In the past century, the root cause of indirect manufacturing costs has changed from a single cost driver (such as direct labor hours) to several cost drivers. Due to sophisticated manufacturing and increased demands from customers, direct labor is no longer the main cost driver of indirect manufacturing overhead.
In addition to direct labor, today's drivers of indirect manufacturing costs include the number of machine setups required, the number of engineering change orders, the demands from customers for special inspections, handling and storage, the number of components in the units produced, and the number of production machine hours.
Manufacturers that want to know the true costs of their products need to know what is driving their indirect manufacturing costs. For these companies it is not sufficient to merely spread overhead costs to products by using a single factor such as direct labor hours or production machine hours.