6 Inventory Management

Inventory’s role and trade-offs

Inventory management determines what to stock, how much to hold, and when and how to replenish it. Its goal is to meet production and customer needs without tying up excessive cash or space. Too little stock can cause shortages and delays; too much increases storage, handling, financing, and obsolescence costs.

Inventory can smooth production, decouple operations, and help meet demand quickly. These benefits must be weighed against ordering or setup costs, carrying costs, and shortage costs such as lost sales, backorders, or interrupted production. A sound policy considers these costs alongside the service customers expect.

Inventory types and purposes

Inventory is a supply of items held for future use or sale. Common categories include:

• Raw materials and components: Items used to make products.

• Work in process (WIP): Items being transformed or waiting between operations.

• Finished goods: Items ready for sale or delivery.

• MRO supplies: Maintenance, repair, and operating items that support operations.

Inventory can also be classified by purpose. Cycle stock covers expected demand between replenishments. Safety stock buffers against uncertainty. Anticipation stock is accumulated ahead of predictable peaks, such as seasonal demand or a planned shutdown. Pipeline inventory is in transit or otherwise tied up in the replenishment process.

Replenishment systems

A replenishment policy specifies when to order and how much to order. Inventory position accounts for stock already available, stock already ordered, and demand that has not yet been filled:

This measure matters because stock already ordered may satisfy future demand, while backorders represent demand not yet filled.

Continuous review and reorder points

In a Continuous-review system, inventory position is monitored continuously or after each transaction. When it reaches the reorder point (ROP), the organization orders a set quantity, $Q$, often an economic order quantity or a practical pack size.

Lead time is the time between placing an order and having the replenishment available for use. For example, if demand averages $12$ units per day, lead time is $5$ days, and safety stock is $20$ units, then:

The order is triggered when inventory position falls to $80$ units.

Periodic review and order-up-to levels

In a Periodic-review system, stock is checked at fixed intervals, such as every Monday. At each review, the order quantity varies: enough is ordered to raise inventory position to a target called the order-up-to level. Because demand can occur between reviews, the target must cover demand during both the review interval and replenishment lead time, plus any safety stock.

Periodic review is convenient when many items are ordered together, but it generally needs a larger buffer than continuous review for the same service goal.

Pull replenishment

Pull replenishment uses actual consumption to trigger replenishment upstream. In a kanban system, a card, bin, or electronic signal authorizes movement or production of a defined quantity. Pull systems connect replenishment to consumption and require reliable signals and disciplined execution.

Safety stock and uncertainty

Safety stock is extra inventory held to protect against uncertainty in demand, forecast error, or supplier lead time. More safety stock usually reduces the chance of a shortage, but increases average inventory and its associated costs. The appropriate amount depends on the variability a business faces and the service level it intends to provide.

When demand varies but lead time is fixed, a common statistical estimate is:

Here, $z$ is the service-level factor, $\sigma_d$ is the standard deviation of demand per time period, and $L$ is lead time in the same time units. The formula assumes demand variation is reasonably modeled by a normal distribution and that periods are independent. If lead time also varies, or demand is seasonal or intermittent, the calculation should account for those conditions rather than treating all uncertainty as ordinary daily demand variation.

For example, if daily demand has a standard deviation of $3$ units, lead time is $4$ days, and the chosen service-level factor is $1.65$, estimated safety stock is:

This is about $10$ units. If average demand is $12$ units per day, the corresponding reorder point is:

This estimate is not a universal target. A higher service goal raises the buffer, and different shortage consequences may justify different targets across items.

Inventory control and performance

Inventory control keeps recorded quantities, physical stock, and operating decisions aligned. Effective practices include:

1. Maintain accurate records. Record receipts, issues, returns, and adjustments promptly; use a perpetual inventory system where appropriate.

2. Classify items by importance. ABC analysis groups items by annual dollar usage, calculated as $\text{annual demand} \times \text{unit cost}$. A-items typically receive tighter monitoring and more frequent review; C-items may use simpler controls. Value is not the only consideration: a low-cost item that can stop production may also need close attention.

3. Set and review replenishment parameters. Reassess order quantities, reorder points, lead times, and safety stock when demand, supplier performance, or service needs change.

4. Verify physical stock. Cycle counting checks selected items on a planned schedule and investigates discrepancies instead of relying only on an occasional full count.

5. Use suitable storage and rotation rules. Clear identification, location control, and appropriate first-in-first-out (FIFO) handling can reduce losses; perishable goods may require first-expired-first-out handling.

6. Monitor performance. Track measures such as inventory accuracy, stockout frequency, service level, turnover, and obsolete or excess stock. A single metric is not enough: high turnover, for example, is not beneficial if it comes with frequent shortages.

Inventory supports production and customer service, but it consumes resources and can become a source of waste. Continuous review triggers a fixed order at a reorder point; periodic review orders up to a target at set intervals; and pull systems replenish in response to actual consumption. Safety stock buffers uncertainty, while accurate records, sensible item classification, physical verification, and performance review help inventory policies work in practice.