One activity base must be selected for each responsibility center.Fixed expenses are time related and, for flexible budget purposes, are related to a short time span such as a month. In contrast, variable expenses must be related to an activity base in each responsibility center that realistically measures the output or productive activity of the center. In the simple case of a responsibility center that produces one kind of output, variable expenses can be related to units produced as the activity base. For example, if the power department produces only kilowatt of electricity, power department variable expenses can be related to kilowatt hours because this activity base would realistically measure the output of the department. However, in the case of a department producing more than one type of output, such as two or more dissimilar products, some equivalent measure of the different outputs must be selected as the activity base.For example, one department may use direct labor hours, another department may use direct machine hours, and repair department may use direct repair hours their respective activity bases. The activity base selected must be the one that most realistically measures the overall activity of the responsibility center