4 Inventory Planning and Control

The role of inventory

Inventory planning determines how much stock to hold, where to position it, and when and how to replenish it. Its central challenge is balancing availability for customers and operations against the cash and risk tied up in stock.

Why organizations hold stock

Organizations hold inventory for several distinct operational purposes:

• Cycle stock covers expected demand between replenishment orders.

• Safety stock buffers uncertainty in demand, supply, or replenishment time.

• Pipeline inventory is stock in transit or otherwise moving through production and distribution.

• Decoupling inventory lets one process or work center continue operating when another is temporarily interrupted.

• Anticipation inventory is built ahead of expected seasonal peaks, planned shutdowns, or other known events.

Inventory may consist of raw materials, work in process, finished goods, or maintenance, repair, and operating supplies. Each type supports a different operational need. Inventory can protect service and production continuity, but it consumes space and working capital; excess stock can increase costs and become obsolete or spoil.

Inventory costs and trade-offs

Managers commonly distinguish four cost categories:

• Purchase cost: the amount paid for an item, including relevant discounts and inbound costs.

• Ordering or setup cost: the administrative, receiving, and transportation costs of placing and processing an order. In manufacturing, a production setup may be the analogous cost.

• Carrying cost: the cost of holding stock, including capital tied up, storage, insurance, handling, damage, spoilage, and obsolescence.

• Shortage cost: the consequences of insufficient stock, such as lost sales, backorders, production stoppages, or customer dissatisfaction.

Larger, less frequent orders can reduce the number of orders placed but increase average cycle stock. Smaller, more frequent orders can reduce stock on hand but may raise ordering, setup, or transportation costs. Planning therefore seeks a suitable balance among availability and costs rather than simply minimizing inventory.

Choosing a replenishment policy

A replenishment policy specifies when to order and how much to order. Two common approaches are the Continuous-review policy and the Periodic-review policy.

Continuous review and reorder points

In a Continuous-review policy, commonly written as $(Q, R)$, inventory position is monitored as transactions occur. When inventory position reaches the Reorder point $R$, the organization orders a set quantity $Q$.

For example, if demand averages $20$ units per day, lead time is $5$ days, and safety stock is $25$ units, expected lead-time demand is $100$ units and the reorder point is $125$ units. When inventory position reaches $125$, the organization places an order for $Q$ units. The order quantity may be based on an economic order quantity, supplier constraints, or an agreed lot size. The reorder point determines when to order; by itself, it does not determine how much to order.

Periodic review and other ordering rules

In a Periodic-review policy, commonly written as $(P, S)$, stock is checked at fixed intervals $P$. At each review, the organization orders enough to raise inventory position to a target level $S$. Because demand can occur between reviews, the policy must protect against demand over both the review interval and replenishment lead time. Periodic review can be convenient when several items are ordered from the same supplier on a regular schedule.

Other practical rules include min–max replenishment, which triggers an order when stock falls to a minimum and replenishes toward a maximum, and lot-for-lot ordering, which plans supply to match specific requirements. The best choice depends on demand patterns, item value, supplier lead time, order constraints, and the cost of monitoring stock.

Safety stock and uncertainty

Safety stock protects against uncertainty; it is not intended to cover ordinary expected demand. The required amount generally increases when demand or lead time becomes more variable, or when the desired service level rises.

When lead-time demand is approximately normally distributed, a common calculation is:

Here, $z$ is selected for the desired Cycle service level. For example, if the standard deviation of lead-time demand is $15$ units and the target cycle service level is $95\%$, $z$ is approximately $1.645$. Safety stock is therefore about $25$ units, calculated as $1.645 \times 15$ and rounded. Adding it to expected lead-time demand gives the reorder point. This method depends on its assumptions: planners should use observed demand and lead-time data and review the estimate as conditions change.

Defining service levels

The phrase service level can refer to different measures, so the chosen metric should be stated clearly.

• Cycle service level (CSL) is the proportion or probability of replenishment cycles with no stockout.

• Fill rate is the proportion of total demand supplied immediately from available stock.

These measures are not interchangeable. A cycle with even a small shortage counts as a stockout for cycle service level, while fill rate reflects how many units were actually supplied. A higher target can improve availability but usually requires more safety stock and carrying cost.

Controlling inventory in practice

Inventory control keeps physical stock, records, and planned replenishment aligned. Reliable decisions depend on accurate item, location, quantity, and transaction data. Consistent controls help ensure that system information reflects actual stock movements.

• Record receipts, issues, transfers, returns, and adjustments promptly.

• Use barcodes or other scanning methods to reduce manual-entry errors.

• Use Cycle counting to count selected items regularly, and investigate discrepancies rather than relying only on an annual count.

• Apply ABC classification to focus attention: A items typically receive tighter monitoring, while lower-value C items may use simpler controls. ABC classification is a prioritization aid, not a substitute for considering criticality, demand variability, or supply risk.

• Track inventory by lot, serial number, or expiry date when traceability or product rotation requires it.

• Review exceptions such as stockouts, excess or obsolete stock, late supplier deliveries, and unexplained count differences.

Warehouse and inventory systems support receiving, put-away, locating stock, replenishment, and cycle counting. Their information is useful only when physical movements are captured accurately and controls are followed consistently.

Putting inventory planning together

Inventory supports customer service, production, and operational continuity, but ties up capital and creates carrying, ordering, and shortage costs. Replenishment policies determine when and how much to order; reorder points combine expected lead-time demand with safety stock. Safety stock reflects uncertainty and a clearly defined service target, while accurate records, regular counts, item prioritization, and exception review help make the plan work in practice.