Inventory Management



To minimize supply and demand imbalances in the supply chain, firms utilize various methods of inventory management. The problem is complicated by the fact that demand is uncertain, and this uncertainty can cause stockouts in which inventory is depleted and orders cannot be filled.

Here, we discuss a model in which the inventory level is reviewed periodically, and orders are placed at regular intervals to order up to a certain base stock. This policy is known as a Policy of Periodic Review, Order-Up-To Base Stock.

Under this policy, one orders a variable quantity  Q  every fixed period of time  p  in order to maintain an inventory position ( Qty on hand  +  Qty on order ) at a predefined base stock level  S,  also known as the “order-up-to level.” The base stock level  S  is determined by calculating the quantity needed between the time the order is placed and the time that the next period’s order is received, and adding a quantity of safety stock to allow for variation in the demand.

The time between the placing of the order and the receiving of the next period’s order is the sum of the review period  p  and the replenishment lead time  l  (lower-case L). The demand per unit of time,  μ , is multiplied by the time between order placement and the next period’s order arrival ( p + l ) to determine the expected quantity to be sold. The safety stock depends upon the variability in the demand and the desired order fill rate.

To calculate the safety stock, first calculate the standard loss function, designated as L(z). This function is dependent on the values of the desired fill
rate  f,  the demand  μ  and its standard deviation  σ ,  the time between orders  p,  and the replenishment lead time l :

L(z)  =  ( 1 – f ) µ p / σ  ( p + l )1/2

Once L(z) is known, z can be found in a look-up table and the safety stock can be calculated by:

Safety Stock  =  z σ ( p + l )1/2

If the review period  p  is reduced, the safety stock does not necessarily reduce because  p  is in both the numerator and denominator of the standard loss function which determines the value of  z.

The average level of on-hand inventory is the sum of the cycle stock ( equal to  µp/2 ) and the safety stock. The on-hand inventory does not include those units in the delivery pipeline.

This model can be complicated by the following real-world issues: variable lead times, non-stationary demand, multiple inventory sites, multiple customer classes, and multi-item order fill rate.

When several components are needed to build a system, each component having the same fill rate, the overall system order fill rate (multi-item fill rate) will be lower than the component fill rate since an order cannot be completed even if only a single component is missing. The multi-item fill rate is the product of the individual item fill rates. For n items having the same component fill rate:

order fill rate  =  (component fill rate)n

When there are long shipping times, the idea of postponing the last stages of final assembly until the product reaches the distribution center (DC) may become attractive. At the DC, the units can be localized and customized according to the demand patterns seen at that time. The result is that the total safety stock required at the DC is reduced by a factor of n, where n is the number of different SKU’s for which the customization is being postponed.

To maximize the benefits of postponement, the product should be designed to be distribution center localizable. The variable features of the product can be isolated into one or two modules that are to be installed in the distribution center.