The aggregate planning approach utilizes an aggregate unit of production. This unit can be considered an average item if the items are similar, but if there are many different types of items, it is more appropriate to consider aggregate units in terms of weight (tons of aluminum), volume (gallons of gasoline), amount of work required (worker-years of military training), or dollar value (value of inventory in dollars). The appropriate scheme to use is not always obvious and depends on the context of the problem as well as the level of aggregation required.