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).