Q

Order Quantity (Q) （注文数）

Q is an output variable (Reports/Output by Site) that represents the optimal number of a part to be ordered at a location from the next higher echelon in a single transaction. The value depends on the relationship between demand rate, the cost of holding inventory and the cost of ordering at the location.

During the optimization, each item is treated as if one-for-one replenishment (repair or resupply) of each demand is appropriate. For each item and location, the stock level is a constant and it equals on-hand plus on-order minus backorders. uses the expected backorders per item from the optimization as a constraint, and computes the optimal order quantity, Q, and reorder point, R, using the order cost at that echelon and the holding cost rate. Note that the only backorders that contribute directly to availability are those on 1st-indenture items that occur at operating sites. The others contribute only indirectly by delaying resupply or repair. 

The demand for an item at any echelon comprises two portions: some that are repairable at that same location, some that must go to the next higher echelon (items are condemned or scrapped only at the highest echelon of the organization where demand on the next higher echelon is a procurement). An order for Q units should be placed when the stock on hand plus stock in repair locally plus the stock already on order falls below the reorder point R. It is for this reason that the item detail output displays the average on hand plus amount in local repair at each site.

A value of Q = 1 corresponds to a one-for-one replenishment policy (where the stock level s corresponds to R + Q and the reorder point R = s - 1). When the stock level is zero, Q = 1 and R = - 1, indicating that we reorder only when a backorder occurs at that location for the item. The values of Q tend to be larger at the depot where demand is largest, lead times are longest and ordering costs highest. For very low cost items, the value of Q may become quite large; for this reason, the maximum value of Q is set to five years of demand.

For any item with a Q = 1, indicating one-for-one replenishment, the stock levels should be used. Only for those items and locations with a Q larger than 1 would the values of Q and R be used instead, since two decision variables are needed.

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