5 Inventory Management

Inventory’s purpose and trade-offs

Inventory management determines how much stock to hold, where to hold it, and when and how much to replenish. Its aim is to make items available when needed while controlling ordering, storage, handling, and stockout costs.

Too little stock can interrupt production or result in lost sales. Too much stock ties up cash and warehouse space and may become obsolete. The appropriate amount depends on the item's importance, demand pattern, lead time, supply reliability, and the cost of a shortage.

Roles inventory can serve

Organizations hold inventory for several operational purposes:

• Cycle stock covers normal demand between replenishment orders. Larger, less frequent orders generally increase cycle stock.

• Safety stock buffers against uncertain demand or replenishment lead time.

• Anticipation stock is built ahead of expected peaks, such as seasonal demand or a planned shutdown.

• Pipeline stock is inventory in transit or otherwise moving through the supply chain.

• Decoupling stock separates process stages so a delay or interruption at one stage does not immediately stop another.

These roles can overlap: the same units may support normal demand and provide some protection against uncertainty. The appropriate amount depends on the item's importance, demand pattern, lead time, supply reliability, and the cost of a shortage.

Inventory position and replenishment

A replenishment policy specifies what inventory measure to monitor, when to order, and how much to order. Decisions usually use inventory position, rather than only the quantity physically on the shelf:

This measure accounts for stock already committed to customers as well as stock expected from suppliers.

Continuous-review policies

In a continuous-review (s, Q) policy, inventory position is monitored continually. When it reaches or falls below the reorder point $s$, the organization orders a fixed quantity $Q$. The reorder point covers expected demand during supplier lead time plus any safety stock:

For example, if average demand is $20$ units per day and lead time is $4$ days, expected lead-time demand is $80$ units. With $17$ units of safety stock, the reorder point is $97$ units. An order is triggered when inventory position reaches $97$, not necessarily when on-hand stock does.

An $(s, S)$ policy also triggers at $s$, but orders enough to raise inventory position to a target $S$, rather than ordering a fixed quantity.

Periodic-review policies

In a periodic-review (R, S) policy, inventory position is checked at regular intervals $R$, and an order raises it to the target $S$. Because stock is not checked continuously, the target must protect against demand during both the review interval and supplier lead time.

Periodic review can coordinate orders or simplify administration, but it may require more buffer stock than continuous review.

Safety stock and uncertainty

Safety stock protects against uncertainty; it is not a substitute for correcting avoidable problems such as inaccurate records or unreliable replenishment. A common calculation assumes normally distributed demand, independent demand across periods, and fixed lead time:

Here, $z$ is the standard-normal factor for the chosen cycle service level, $\sigma$ is the standard deviation of demand per period, and $L$ is lead time in the same periods. With daily demand standard deviation of $5$ units, a lead time of $4$ days, and a $95\%$ cycle service level, the calculation gives approximately $1.645 \times 5 \times \sqrt{4} = 16.45$, or about $17$ units. The corresponding reorder point is average demand during lead time plus those $17$ units.

If lead time varies, its variability should also be included in the demand uncertainty calculation. The simple formula above does not account for lead-time variability.

Service measures and inventory cost

A cycle service level (CSL) is the probability of avoiding a stockout during a replenishment cycle. A fill rate is the share of total demand supplied immediately from available stock. These measures are related but not interchangeable: a high probability of having no stockout in a cycle does not by itself specify what fraction of units will be filled from stock.

The service measure and target should match the business objective. Raising the target generally requires more safety stock and increases holding costs, so managers balance availability against the cost of inventory.

Inventory control in practice

Inventory control puts the replenishment policy into practice and helps keep recorded quantities dependable.

• Maintain accurate transactions: promptly record receipts, picks, returns, transfers, adjustments, and items in transit.

• Count and reconcile: use cycle counting for regular checks, and investigate the causes of discrepancies rather than merely adjusting balances.

• Prioritize attention: use ABC analysis to group items by a chosen measure of importance, often annual usage value. A items receive tighter monitoring; lower-priority items may use simpler controls. ABC ranking should not override criticality, because a low-value spare can still be essential.

• Control physical stock: use clear item identification and storage locations; apply lot, serial-number, or expiry-date tracking where needed; and follow appropriate rotation rules for dated goods.

• Review settings and performance: update demand and lead-time estimates, reorder points, order quantities, and service targets as conditions change. Track measures such as stockouts, fill rate, record accuracy, excess or obsolete stock, and inventory value.

Reliable control links purchasing, receiving, warehousing, and fulfillment. Inaccurate stock records can cause both unnecessary orders and missed replenishment, so inventory decisions should be reviewed as part of the end-to-end flow of goods rather than as isolated warehouse settings.