Inventory Planning and Control

Introduction

Inventory is the stock of materials, components, work in process, finished goods, spare parts and supplies held to support operations and customer service. It connects purchasing, production, distribution and sales. Inventory planning determines what to stock, how much to stock and when to replenish it. Inventory control ensures that actual stock remains aligned with those decisions.

The central managerial problem is a trade-off. Too little inventory causes stockouts, production interruptions, emergency purchases and lost sales. Too much inventory ties up capital, consumes space, increases handling and insurance costs, and creates risks of damage, deterioration and obsolescence. Effective control therefore aims not at maximum inventory or minimum inventory, but at the economically appropriate level of availability.

Learning Objectives

After studying this chapter, the learner should be able to:

• classify inventory according to its purpose and position in the supply chain;
• apply ABC analysis to prioritize control effort;
• calculate economic order quantity, reorder point and safety stock under stated assumptions;
• distinguish continuous-review and periodic-review systems;
• explain service level, MRP, JIT and cycle counting;
• assess the cost, availability and obsolescence consequences of inventory policies;
• use technology and governance practices to improve inventory accuracy.

1. Nature and Classification of Inventory

Inventory exists because supply and demand rarely occur at exactly the same time, quantity or location. It performs several functions:

• Transaction or cycle stock: inventory required because replenishment occurs in batches rather than one unit at a time.
• Safety stock: additional inventory held against demand, lead-time or supply uncertainty.
• Anticipation stock: inventory accumulated before predictable events such as seasonal demand, promotions, strikes or planned price increases.
• Pipeline inventory: goods in transit or otherwise moving between supply-chain stages.
• Decoupling stock: inventory that permits adjacent processes to operate independently when their rates or schedules differ.
• Speculative stock: inventory purchased to benefit from expected price increases or to protect against known scarcity.

Inventory may also be classified by its physical position:

A useful distinction is between independent demand and dependent demand. Independent demand is determined externally by customers and is forecast or monitored, as with finished products. Dependent demand is derived from the requirements of another item, as with components in a product structure. Independent-demand items are commonly controlled through reorder-point or periodic-review systems; dependent-demand items are often planned through MRP.

The inventory record should identify item code, description, unit of measure, location, supplier, lead time, standard or actual cost, lot size, demand history, expiry information and status. Confusing units—for example, ordering cartons while counting pieces—can create apparently unexplained shortages.

2. Inventory Costs and Performance Measures

Four cost categories are particularly important:

1. Purchase or production cost: the price or conversion cost of the item.
2. Ordering or setup cost: administrative, transport, inspection and receiving costs incurred per replenishment order or production run.
3. Holding cost: capital, storage, insurance, handling, shrinkage and deterioration costs.
4. Shortage cost: lost contribution, expediting, production downtime, penalties and loss of goodwill.

Annual holding cost per unit is often represented as H = iC, where i is the annual holding-rate fraction and C is unit value. If i = 0.20 and C = $50, then H = $10 per unit-year. The rate should reflect the organization’s actual economic costs rather than an arbitrary percentage.

Common performance indicators include inventory turnover and days of supply:

Turnover is meaningful only when valuation and time periods are consistent. A high turnover rate is not automatically desirable if it causes unacceptable stockouts; a low rate may be appropriate for critical spare parts or long lead-time materials.

The fill rate is the proportion of demand supplied immediately from stock. The cycle service level is the probability that no stockout occurs during a replenishment cycle. These measures are related but not identical: a cycle may experience a small shortage and still have a high fill rate, while a cycle service measure treats any stockout as a failure.

3. ABC Analysis and Inventory Prioritization

ABC analysis applies the Pareto principle to annual usage value. For each item:

Items are ranked from highest to lowest annual usage value, and cumulative percentages are calculated. A typical policy is:

The percentages are not universal. An organization may define A as the first 70–80% of annual usage value, B as the next 15–25%, and C as the remainder. ABC is a prioritization tool, not a statement that C items are unimportant. A low-value fire extinguisher part may be operationally critical.

For this reason, ABC should be combined with other classifications. VED classifies items as vital, essential or desirable according to shortage consequences. FSN classifies movement as fast, slow or non-moving. HML uses high, medium or low unit price. XYZ may classify demand predictability or inventory-value variability. A matrix such as AV, AE or CV can direct attention more effectively than ABC alone.

Worked Example: ABC Classification

Suppose four items have the following annual demand and unit costs:

Total annual usage value is $106,000. The ranked cumulative percentages are P: 47.2%, Q: 75.5%, R: 92.5% and S: 100%. Under a policy defining A as approximately the first 70–80%, P and Q are A items; R is B; S is C. Management should devote more forecasting, review and cycle-counting effort to P and Q, while using simpler controls for S.

4. Economic Order Quantity

The basic EOQ model identifies the order quantity that minimizes annual ordering plus holding cost when demand is relatively stable.

Here, Q* is economic order quantity in units per order, D is annual demand in units per year, S is ordering or setup cost per order, and H is annual holding cost per unit. If holding cost is a rate, H = iC.

The model assumes constant known demand, constant lead time, instantaneous replenishment, no stockouts, no quantity discounts, and ordering cost independent of order size. These assumptions make EOQ a benchmark, not a command. Capacity, supplier minimums, transport economics, shelf life and quantity discounts may require adjustment.

The annual relevant cost is:

The first term is annual ordering cost and the second is annual cycle-stock holding cost. Differentiating this expression with respect to Q and setting the derivative to zero gives EOQ; at the optimum, ordering cost equals cycle-stock holding cost.

Worked Example: EOQ

Annual demand is 12,000 units, ordering cost is $40 per order and annual holding cost is $2 per unit.

Orders per year are 12,000 ÷ 693 ≈ 17.3, and the average cycle stock is approximately 693 ÷ 2 = 346.5 units, excluding safety stock. If the supplier requires a minimum of 1,000 units, the buyer must compare that constraint with the EOQ and assess additional holding and obsolescence cost.

5. Reorder Point and Safety Stock

A continuous-review system records inventory after each transaction and places an order when the inventory position reaches a reorder point. Inventory position is:

For constant demand and lead time:

ROP is reorder point in units, d is average demand per time unit, L is lead time in the same time unit, and SS is safety stock. The product dL is expected lead-time demand.

If demand varies while lead time is constant, a common statistical approximation is:

Here, z is the standard-normal factor for the selected cycle service level, σd is the standard deviation of demand per time unit, and L is lead time in those time units. This formula assumes independent, approximately normal period demand and known, constant lead time.

If lead time varies but demand is constant:

If both demand and lead time vary independently, a widely used approximation is:

The formula ignores the covariance between demand and lead time. If covariance is material, a more complete variance expression is required. The selected z depends on the target cycle service level: approximately 1.28 for 90%, 1.645 for 95% and 2.33 for 99%, under a normal-distribution assumption.

Worked Example: Reorder Point

Daily demand has mean 100 units and standard deviation 20 units. Lead time is 4 days, and the desired cycle service level is 95%.

Expected lead-time demand is 100 × 4 = 400 units. Therefore:

An order is triggered when inventory position falls to approximately 466 units. This does not guarantee that every demand event will be met; it expresses a probabilistic service objective based on the model assumptions.

6. Periodic-Review Systems and Service Levels

In a periodic-review system, stock is reviewed at fixed intervals P. Each review places an order sufficient to raise inventory position to an order-up-to level S. Protection is required over the review period plus lead time, P + L. For variable demand and constant lead time:

The order quantity at a review is:

subject to practical constraints such as minimum order quantities and pack sizes. Periodic review is convenient where many items are ordered together or where transaction monitoring is expensive, but it permits greater stock fluctuation and can expose the firm to demand surges between reviews.

Service level must be selected by business consequence. High targets are appropriate for vital production components or emergency medical supplies, but a 99.9% target for every low-value item may create excessive stock. Target setting should consider shortage cost, customer promise, supplier reliability, item criticality, value and obsolescence risk.

7. Material Requirements Planning

MRP is a time-phased planning system for dependent demand. Its principal inputs are:

1. the master production schedule (MPS), which states what finished products are required and when;
2. the bill of materials (BOM), which specifies components and quantities;
3. inventory records, including on-hand balance, scheduled receipts, lead times, lot-sizing rules and safety stocks.

MRP proceeds by exploding the BOM, offsetting component requirements by lead time, and calculating net requirements. In simplified form:

A planned order receipt is then sized according to the lot-sizing rule, and the planned order release is offset backward by lead time. MRP is powerful when schedules, BOMs, inventory records and lead times are accurate. It is not a substitute for disciplined master scheduling. Nervousness—frequent changes to planned orders—can be reduced through time fences, firm planned orders, accurate data and appropriate planning horizons.

MRP and reorder-point systems can coexist. MRP generally plans components whose demand is derived from production, while reorder-point logic may control consumables, service parts or independent-demand items.

8. Just-in-Time and Lean Inventory

Just-in-time (JIT) seeks to provide the required item, quantity and time with minimal waste. It uses pull signals, small lots, short setup times, reliable equipment, standardized work, quality at source and close supplier relationships. Kanban cards or electronic signals authorize replenishment based on actual consumption rather than a long-range forecast alone.

JIT does not mean zero inventory under all conditions. It reduces unnecessary buffers by removing causes of variability. If suppliers are unreliable, processes are unstable or quality is poor, aggressively reducing inventory can transfer risk into stockouts and disruption. A responsible JIT program therefore combines supplier development, preventive maintenance, quality control, contingency planning and risk-based buffers.

9. Cycle Counting and Inventory Accuracy

Cycle counting verifies selected items throughout the year rather than relying only on an annual physical count. A common policy counts A items most frequently, B items at an intermediate frequency and C items least frequently. Counting frequency should also reflect criticality, transaction errors, theft exposure and regulatory requirements.

A simple record accuracy measure is:

For example, if 94 of 100 counted records are within the approved tolerance, accuracy is 94%. Organizations should define tolerance carefully; a one-unit difference may be serious for a high-value item but immaterial for bulk materials.

A sound cycle-count process freezes or controls transactions, counts independently of the book balance, investigates root causes, approves adjustments and documents corrective action. Causes commonly include incorrect receipts, unrecorded issues, wrong locations, unit-of-measure errors, damage, theft and unauthorized substitutions. Blind counting—where the counter does not see the system quantity—reduces confirmation bias.

10. Technology, Governance and Obsolescence

Enterprise resource planning systems integrate purchasing, inventory, production and finance. Barcodes and radio-frequency identification improve transaction speed and traceability. Warehouse-management systems support location control, picking and replenishment. Forecasting and analytics can identify unusual demand, excess stock and likely stockouts.

AI and machine-learning tools may improve forecasts by detecting seasonality, promotions and external signals, but they do not correct poor master data. Human review remains necessary for product launches, structural breaks, ethical use of data and exceptional events. Automated recommendations should be auditable and subject to approval limits.

Obsolescence is especially important for technology products, fashion goods, chemicals and regulated materials. Indicators include declining demand, engineering changes, expiry dates, supersession and inactive stock. Controls include last-time-buy analysis, phase-out plans, alternate use, supplier return agreements, smaller lots and periodic excess-and-obsolete reviews. Inventory decisions should also respect safety, environmental and legal requirements for storage, disposal, hazardous materials and product traceability.

Applied Case: Meridian Pumps (Illustrative)

Background

Meridian Pumps manufactures industrial water pumps. It experienced frequent shortages of motor bearings despite holding a large total inventory. An initial review showed 2,400 stock-keeping units, inconsistent units of measure and infrequent counting of critical parts.

Problem and Analysis

The bearing was an A item by annual usage value and a V item because its absence stopped assembly. Daily demand averaged 100 bearings, with a standard deviation of 20. Supplier lead time was four days. The company had been ordering monthly, so shortages occurred between reviews. Its recorded inventory was also inaccurate because receipts were posted in boxes while issues were posted in pieces.

Approach and Solution

Meridian standardized the unit of measure, introduced barcode receiving and issuing, and classified items using ABC–VED analysis. For the bearing it adopted continuous review. At a 95% cycle service level, safety stock was calculated as approximately 66 units and the reorder point as 466 units. The company set a practical order quantity near the EOQ but negotiated a supplier pack-size constraint. A cycle-count schedule counted the bearing weekly, and supplier performance was reviewed using lead-time and defect data.

Outcome

After three months, record accuracy for critical items increased, emergency purchases declined and assembly interruptions were reduced. Average inventory did not fall immediately because the firm first corrected records and stabilized supply. This illustrates that availability, cost and accuracy must be improved together; reducing stock before correcting process causes would have increased risk.

Learning Points

• Classification should combine financial value with operational criticality.
• Reorder points require consistent units and realistic lead-time data.
• Technology creates value only when processes and master data are disciplined.
• Safety stock is a service decision, not merely an accounting excess.

Challenges and Best Practices

Inventory models can mislead when demand is intermittent, lead times are correlated with demand, forecasts are biased, or costs are omitted. EOQ may be unsuitable for perishables, severe capacity limits or large quantity discounts. Normal-distribution safety-stock formulas may be poor for lumpy demand. Managers should therefore validate assumptions, use historical error analysis, distinguish forecast uncertainty from demand variability, and review parameters regularly.

Best practice includes: segmenting items by value and criticality; maintaining accurate BOMs and lead times; setting service targets by consequence; integrating purchasing, production and sales; using cycle counting and root-cause analysis; monitoring excess and obsolete stock; involving suppliers; and reviewing policies after promotions, product changes or disruptions.

Chapter Summary

Inventory planning balances service, cost, capacity and obsolescence. Inventory may be held as cycle, safety, anticipation, pipeline, decoupling or speculative stock, and may be classified as raw material, work in process, finished goods, MRO or spare parts. ABC analysis prioritizes control by annual usage value but should be combined with criticality and movement classifications. EOQ balances ordering and holding costs under restrictive assumptions. Reorder points combine lead-time demand with safety stock, while safety stock reflects uncertainty and the selected service level. Periodic review raises inventory position to an order-up-to level; continuous review triggers replenishment at a reorder point. MRP plans dependent demand from the MPS, BOM and inventory records. JIT reduces waste through pull systems and process reliability, while cycle counting protects inventory-record accuracy. Technology, governance, master-data discipline and obsolescence management are essential to sustainable control.

Key Terms

Inventory: Stock held to support supply, production or customer demand.

Cycle stock: Inventory created by ordering or producing in batches.

Safety stock: Buffer inventory held against uncertainty.

Lead time: Time between placing an order and receiving usable material.

Independent demand: Demand determined externally by customers or markets.

Dependent demand: Demand derived from the need for another item.

ABC analysis: Ranking items by annual usage value.

VED analysis: Classification by vital, essential and desirable shortage consequence.

EOQ: Order quantity that minimizes basic ordering and holding costs.

Ordering cost: Cost incurred each time replenishment is placed or set up.

Holding cost: Cost of carrying inventory over time.

Reorder point: Inventory position at which replenishment is triggered.

Inventory position: On-hand plus on-order inventory minus backorders.

Service level: Probability or proportion of demand supplied, depending on definition.

Periodic review: Inventory system reviewed at fixed time intervals.

MRP: Time-phased planning system for dependent demand.

JIT: Pull-oriented approach seeking timely supply with minimal waste.

Cycle counting: Continuous partial counting of inventory records.

Obsolescence: Loss of usefulness or value due to age, change or expiry.

Master production schedule: Time-phased plan for finished-product production.

Review Questions

Short Answer Questions

1. Distinguish cycle stock from safety stock.
2. What is the purpose of ABC analysis?
3. Define inventory position.
4. State two inputs required by MRP.
5. Why is cycle counting used?

Long Answer Questions

1. Explain the major inventory cost categories and their effect on policy decisions.
2. Describe the EOQ model, including its assumptions and limitations.
3. Compare continuous-review and periodic-review systems.
4. Explain how MRP converts a master production schedule into component plans.
5. Discuss why JIT requires process and supplier reliability.

Analytical / Application Questions

1. Annual demand is 18,000 units, ordering cost is $30 per order and annual holding cost is $1.50 per unit. Calculate EOQ and explain one practical reason the chosen order quantity might differ.
2. Daily demand averages 80 units with a standard deviation of 15 units, lead time is five days and the desired cycle service level is 95%. Calculate safety stock and reorder point using the constant-lead-time formula.
3. A company has high total inventory but repeated stockouts of critical low-value parts. Design a classification, review and cycle-counting policy to address the problem.

Multiple Choice Questions with Answers

Question 1

Which inventory is held primarily to protect against uncertainty in demand or lead time?

A. Cycle stock

B. Safety stock

C. Pipeline inventory

D. Speculative stock

Correct Answer: B. Safety stock

Question 2

Annual usage value is calculated as:

A. Annual demand ÷ Unit cost

B. Unit cost ÷ Annual demand

C. Annual demand × Unit cost

D. Annual demand + Unit cost

Correct Answer: C. Annual demand × Unit cost

Question 3

Using the basic EOQ assumptions, if annual demand is 10,000 units, ordering cost is $50 and annual holding cost is $2, EOQ is approximately:

A. 224 units

B. 500 units

C. 707 units

D. 1,000 units

Correct Answer: C. 707 units

Question 4

If average daily demand is 50 units, lead time is 6 days and safety stock is 40 units, the reorder point is:

A. 90 units

B. 260 units

C. 300 units

D. 340 units

Correct Answer: D. 340 units

Question 5

Which item is most appropriately planned through MRP?

A. A finished product with independent customer demand

B. A component whose demand is derived from a production schedule

C. An office stationery item with irregular use

D. A service part with no product structure

Correct Answer: B. A component whose demand is derived from a production schedule

Question 6

In a periodic-review system, an order generally raises inventory position to the:

A. Economic order quantity

B. Reorder point

C. Order-up-to level

D. Annual demand level

Correct Answer: C. Order-up-to level

Question 7

Which statement best describes JIT?

A. It requires maximum safety stock at every stage

B. It eliminates the need for supplier quality management

C. It uses pull signals and seeks to reduce waste and unnecessary inventory

D. It is identical to forecasting all demand accurately

Correct Answer: C. It uses pull signals and seeks to reduce waste and unnecessary inventory

Question 8

A record-accuracy measure of 96 correct records out of 120 counted records is:

A. 80%

B. k?

C. 96%

D. 120%

Correct Answer: A. 80%

Question 9

Why should ABC analysis sometimes be combined with VED analysis?

A. To replace all inventory records

B. To consider shortage criticality as well as usage value

C. To ensure every item has the same service level

D. To calculate lead time automatically

Correct Answer: B. To consider shortage criticality as well as usage value

Question 10

If daily demand standard deviation is 10 units, lead time is 4 days and z = 1.645, safety stock under the constant-lead-time formula is approximately:

A. 16.45 units

B. 20 units

C. 32.9 units

D. 65.8 units

Correct Answer: C. 32.9 units