The selection of an activity base for each service department is often a special problem. Activity bases used in the producing departments should not necessarily be used in the service departments. When service departments,such as the power and repair departments, have reliable measures of activity,these measures should be used. Certain service departments, such as administration, clerical, time and motion, and personnel, generally do not have reliable units of measurement.In such cases, it might be necessary to measure activity in terms of the sales volume, the overall production of the plant, or the output of producing departments that are most frequently served.