With direct labor being reduced and manufacturing overhead increasing, the correlation between direct labor and manufacturing overhead began to wane. A logical response was to begin allocating manufacturing overhead on the basis of machine hours instead of direct labor hours.
Companies also began to create new departments to help manage the changing character of the factories. Production departments such as machining, finishing, and assembling were established. Other departments such as quality control, maintenance, and factory administration were designated as service departments (or production service departments), since these departments served the production departments. The company's costs were contained in the accountant's general ledger, which was organized by departments so as to mirror the organization chart and to provide for budgeting and control. Because some of the production departments used more of some service departments' efforts/costs than others, accountants responded by first allocating the service department costs to the production departments, and then developing manufacturing overhead rates for each of the production departments. These rates were computed by dividing each production department's costs (its own direct costs plus the service departments' costs allocated to it) by its machine hours.
Basing the manufacturing overhead rates on a company's production departments was an improvement over using just one rate for the entire plant—particularly when companies began manufacturing a greater variety of products. Some products being manufactured may have required many machine hours in one department but very few hours in another department, while other products may have used a much different combination of machine hours.
Let's illustrate this method by assuming just two products (X and Y) are being manufactured in a factory that has one service department (Factory Administration, S1) and two production departments (Machining, P1; Finishing, P2)