Inventory control and lean overview

Inventory control (also called stock control) is the management of raw materials, work-in-progress, and finished goods to ensure adequate supply while minimizing costs. Effective inventory control balances the risk of stockouts (running out of stock) against the costs of holding excessive inventory.

Key Terms:

• Buffer stock: Minimum inventory level maintained to prevent stockouts during unexpected demand or supply delays
• Re-order level: The stock level that triggers a new order (formula: Re-order level = Maximum usage × Maximum lead time)
• Re-order quantity: The amount ordered each time stock reaches the re-order level
• Lead time: Time between placing an order and receiving delivery
• Stock rotation: Using oldest stock first (FIFO - First In, First Out) to prevent wastage

Lean production is a systematic approach to minimizing waste without sacrificing productivity. Developed by Toyota, lean focuses on creating maximum customer value with minimum resources.

Key Lean Principles:

• Just-In-Time (JIT): Receiving materials exactly when needed for production, eliminating storage costs
• Kaizen: Continuous improvement through small, incremental changes involving all employees
• Cell production: Organizing work into multi-skilled teams responsible for complete production units

Inventory Holding Costs Formula: Total holding cost = Average stock level × Cost per unit × Holding cost %

Cambridge Key Point: Understand that inventory control involves trade-offs between different costs and risks, not simply minimizing stock levels.

Think of inventory control like managing your phone's battery: you don't want it completely dead (stockout) or constantly at 100% charge (excessive inventory). You maintain a comfortable buffer and recharge at optimal times.

Real-World Application - Supermarket Inventory: Tesco uses sophisticated inventory management systems tracking sales data in real-time. Fresh produce has short shelf life, so buffer stocks are minimal with frequent deliveries. Conversely, tinned goods have longer re-order cycles. During Christmas, re-order levels increase automatically as demand patterns change.

Lean Production in Action - Toyota: Toyota pioneered JIT manufacturing where suppliers deliver parts within hours of production need. A car seat manufacturer located next to Toyota's factory delivers seats as cars move down the production line. This eliminates warehouse costs and reduces waste from outdated designs. Kaizen meetings involve assembly workers suggesting improvements—one worker's idea to reposition a tool saved 3 seconds per car, accumulating to thousands of hours annually.

Fashion Industry - Zara's Lean Approach: Zara revolutionized fashion retail through lean principles. Unlike competitors holding 6-month inventories, Zara produces small batches, monitors sales data, and can design, manufacture, and deliver new items within 2 weeks. This responsiveness reduces unsold inventory by 85% compared to industry averages.

The Trade-off Reality: Amazon maintains enormous warehouses (high holding costs) to guarantee fast delivery, while Dell manufactures computers only after orders are confirmed (JIT approach). Both succeed because they align inventory strategy with customer expectations and business models—there's no universal "best" approach.

Example 1: Calculating Re-order Level

Question: A bakery uses flour with maximum daily usage of 50kg and maximum lead time of 4 days. Calculate the re-order level.

Solution:

• Formula: Re-order level = Maximum usage × Maximum lead time
• Re-order level = 50kg × 4 days = 200kg
• Examiner note: When stock falls to 200kg, the bakery should order more flour

Analysis: This buffer prevents stockouts if demand spikes or delivery delays occur. With 200kg remaining and worst-case consumption of 50kg/day over 4-day delivery, the bakery never runs out.

Example 2: Inventory Costs Comparison

Question: Compare annual costs for two approaches: (a) Current system: average stock 5,000 units, holding cost 15% of unit value (£10), order cost £200 × 12 orders/year; (b) JIT system: average stock 500 units, holding cost 15%, order cost £200 × 52 orders/year.

Solution: Current System:

• Holding cost = 5,000 × £10 × 0.15 = £7,500
• Ordering cost = £200 × 12 = £2,400
• Total = £9,900

JIT System:

• Holding cost = 500 × £10 × 0.15 = £750
• Ordering cost = £200 × 52 = £10,400
• Total = £11,150

Conclusion: JIT costs £1,250 more annually but offers benefits like reduced waste, improved cash flow, and fresher products.

Examiner tip: Always show your workings and explain trade-offs beyond pure costs—Cambridge rewards analytical thinking.