You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Spec mapping: AQA 7138 Unit 3.2.2 — Operations Management (refer to the official AQA specification document for exact wording). This lesson develops inventory management at A-Level depth — the four inventory categories (raw materials, WIP, finished goods, consumables), the inventory-control sawtooth diagram, the JIT vs JIC inventory-philosophy split, the EOQ trade-off between ordering cost and holding cost, ABC analysis for differentiated stock control, the inventory-turnover days KPI (Annex 7 formula 37), and the diagnostic question of how a business should optimise the stockout-vs-holding-cost trade-off under lead-time and demand uncertainty. The 6-mark Analyse tariff calls for rigorous analytical chain-of-reasoning on ONE inventory-management decision — typically the trade-off framing under specific case-study conditions.
Connects to:
Definition: Inventory is the stock of physical goods a business holds at any point — raw materials awaiting processing, work-in-progress (WIP) part-way through production, finished goods awaiting despatch, and consumables (tools, lubricants, office supplies) supporting operations. Inventory management is the operational discipline of deciding how much of each stock category to hold, when to reorder, in what quantity, from which suppliers, under what control systems.
Inventory is simultaneously an asset (it appears on the balance sheet, supports current-ratio liquidity, enables order fulfilment) and a cost (it ties up working capital, occupies warehouse space, incurs insurance and obsolescence loss, hides quality problems in large batches). The strategic question is the optimal level of inventory holding — not the maximum (over-investment in stock) and not the minimum (stockout risk under demand or supply shock).
Four features make inventory management strategically loaded:
| Type | Description | Typical example |
|---|---|---|
| Raw materials | Inputs that have not yet entered production | Steel coil for a sheet-metal fabricator; aluminium ingots for a die-caster; flour and yeast for a baker |
| Work-in-progress (WIP) | Partially completed goods currently being processed | A car partway down the assembly line; a circuit board in a partly-completed batch |
| Finished goods | Completed products ready for sale or despatch | Boxed and palletised consumer electronics in a distribution centre |
| Consumables | Items used in production but not incorporated into the product | Cutting-tool inserts, lubricants, cleaning materials, packaging supplies |
Each category requires different management approaches. Raw-material inventory turns on supplier-relationship and lead-time management; WIP turns on production-flow design and bottleneck management; finished-goods inventory turns on demand-forecasting accuracy and channel-management; consumables typically use simple min-max reorder systems with limited management attention.
The classic inventory-control diagram represents the time-pattern of stock levels under a steady-state reorder system. Key terms:
| Term | Definition |
|---|---|
| Maximum stock level | The highest level the business is willing or able to hold (warehouse capacity, budget, working-capital constraint) |
| Buffer stock (minimum stock level) | The safety margin below which stock should not fall — protects against demand spikes and supply delays |
| Reorder level | The stock level that triggers a new purchase order |
| Reorder quantity | The standard quantity ordered each time |
| Lead time | Time elapsed between placing the order and receiving the delivery |
The sawtooth pattern: stock starts at maximum after a delivery, declines steadily as it is used, reaches the reorder level (a new order is placed), continues to decline during the lead time, then jumps back up to maximum when the delivery arrives. If lead time and usage rate match assumption, the delivery arrives exactly as stock reaches the buffer level. If demand spikes or the delivery is late, the buffer absorbs the variance.
Reorder level = Lead time × Average daily usage + Buffer stock
Worked example. A bakery uses 500 kg of flour per day. Supplier lead time is 3 days. Buffer stock target is 300 kg.
Reorder level = (3 days × 500 kg/day) + 300 kg buffer = 1,800 kg
When flour stock falls to 1,800 kg, the bakery places a new order. During the 3-day lead time, the bakery consumes 1,500 kg (3 × 500), and the delivery arrives just as stock reaches 300 kg (the buffer level), restoring stock to maximum.
The strategic choice of inventory philosophy maps to the lean / traditional split developed in the lean-production lesson.
| Dimension | JIT (lean) | JIC (traditional) |
|---|---|---|
| Inventory level | Minimal or zero | Substantial buffer |
| Production trigger | Pull (confirmed demand) | Push (forecast) |
| Working capital tied up | Low | High |
| Vulnerability to supply shock | High | Low |
| Quality-problem visibility | Immediate | Delayed |
| Cost per unit (in normal times) | Low | Higher (holding cost) |
| Cost per unit (under supply shock) | High (lost production) | Low (buffer protects) |
Post-2020 supply-chain shocks have pushed many firms towards a hybrid inventory policy — JIT for routine inputs, strategic JIC for critical components (semiconductors, lithium, rare-earth elements). The hybrid policy treats criticality as the lens, not a uniform JIT-or-JIC choice across all inputs.
The EOQ is the order quantity that minimises the total cost of inventory ordering and holding. The trade-off is between:
The EOQ is the order size that minimises the sum. The exact EOQ formula (Wilson's lot-size formula) is not examinable at A-Level, but the trade-off intuition is: very small orders minimise holding cost but multiply ordering cost; very large orders minimise ordering cost but inflate holding cost. The optimum balances the two.
The Pareto principle applies to inventory: typically ~20 % of stock items represent ~80 % of stock value. ABC analysis classifies stock items by value to apply differentiated management attention:
| Class | Share of items | Share of value | Management approach |
|---|---|---|---|
| A items | ~15–20 % | ~70–80 % | Tight control: weekly reorder review, supplier-partnership investment, dedicated buyer attention |
| B items | ~25–30 % | ~15–20 % | Moderate control: monthly review, standard reorder systems |
| C items | ~50–60 % | ~5–10 % | Loose control: simple min-max bins, periodic bulk reorders, minimal management attention |
ABC analysis is a cost-effective management attention allocation — there is no point spending senior-buyer time on a £5 cleaning-fluid reorder while a £40k component reorder runs on autopilot.
Inventory turnover (times per year) = Cost of goods sold ÷ Average inventory value (Annex 7 formula 37 — provided in the exam formula sheet)
Inventory days = 365 ÷ Inventory turnover (or equivalently: (Average inventory ÷ Cost of goods sold) × 365)
Inventory turnover (Annex 8 financial concept #c11) is the headline operations/finance synoptic KPI. A high turnover (more times per year) indicates efficient inventory use; a low turnover indicates excess inventory tying up working capital. Sector benchmarks vary widely — a grocer typically turns inventory 20-50 times per year; a high-fashion retailer 6-12 times; a heavy-equipment manufacturer typically 2-5 times; an aerospace contractor 1-3 times. The right benchmark is sector-specific; comparing a heavy-equipment manufacturer against a grocer benchmark is operationally meaningless.
The 7138 paper most often tests this KPI in three ways: (1) calculate inventory days from the financial figures; (2) interpret a change in inventory days as evidence of operational improvement or deterioration; (3) interpret a level of inventory days against a sector benchmark and identify what management action the gap supports. The exam-relevant move is to remember that inventory days is a diagnostic, not a target — the question is what the figure tells us about the business and what management action is justified by the diagnosis.
The diagnostic interpretation is nuanced. Rising inventory days can signal demand softening (sales slow, stock builds), supply-chain disruption (suppliers over-shipping), or strategic JIC build (deliberate strategic-stock build of critical inputs). Falling inventory days can signal lean-programme success (genuine operational improvement), demand strengthening (sales draw down stock), or distress-driven destocking (cash-flow pressure forcing inventory liquidation). The figure in isolation tells you the symptom; the cross-reference to demand, supplier-relationship and cash-flow evidence tells you the cause. Top-band answers do not stop at the symptom.
The interaction with the broader Annex 7 financial KPIs matters. A 10-day reduction in inventory days on a £100m revenue business with a 60 % cost ratio releases approximately £1.64m of working capital — which directly improves the current-ratio (Annex 8 financial concept #c7) and the acid-test ratio (Annex 8 financial concept #c8), and reduces financing cost on any inventory-financed working-capital facility. The released cash can be redeployed to capital investment, debt reduction, or shareholder distribution — the management decision on redeployment is itself part of the strategic value of inventory-discipline programmes.
flowchart TD
Strategy["Operations strategy:<br/>JIT, JIC, hybrid"] --> Categories["Inventory categories:<br/>RM, WIP, FG, consumables"]
Categories --> ABC["ABC analysis<br/>(differentiated attention)"]
ABC --> Controls["Reorder systems:<br/>level, quantity,<br/>lead time, buffer"]
Controls --> EOQ["EOQ trade-off:<br/>ordering vs holding cost"]
Controls --> Buffer["Buffer stock policy:<br/>stockout-vs-holding<br/>cost asymmetry"]
EOQ --> KPIs["Inventory turnover days<br/>(Annex 7 formula 37)"]
Buffer --> KPIs
KPIs --> WorkingCap["Working-capital impact:<br/>current ratio, cash flow"]
KPIs --> Service["Customer-service impact:<br/>OTIF, stockout rate"]
WorkingCap -. iteration .-> Strategy
Service -. iteration .-> Strategy
style Strategy fill:#1d4ed8,color:#fff
style Controls fill:#a16207,color:#fff
style KPIs fill:#15803d,color:#fff
The diagram captures the integrated structure — inventory strategy is downstream of operations strategy, decomposed by category and value (ABC), implemented through reorder controls, measured through KPIs that feed back into working-capital and customer-service outcomes. The dotted feedback loops are critical — inventory strategy is iterative, not one-off.
Tarnford Plumbing Supplies is a hypothetical UK trade-only plumbing-and-heating distributor with 18 branches across the Midlands, established 1996 and employing 162 people. 2025 revenue was £42.3 million; cost of goods sold was £33.4 million; gross margin 21 %; operating profit margin 4.8 %. Average inventory across the 18 branches and the central warehouse was £6.8 million. The new operations director has reviewed the inventory position and recommends a 35-day reduction in inventory days, achieved through three changes: tighter ABC discipline on the C-class slow-movers (40 % of SKUs but only 6 % of stock value), a partnership-supplier renegotiation with the top three brands (covering 71 % of A-class items) for 48-hour replenishment, and a 50 % reduction in branch-level buffer stock on the top 200 fast-moving SKUs.
Figures and company are fabricated for illustrative purposes; not affiliated with any actual business.
Analyse the likely effect on Tarnford Plumbing Supplies of reducing inventory days by 35 days. (6 marks)
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.