interpretation of simple inventory control charts
4.2 Inventory Management – Managing Inventory
What is an Inventory Control Chart?
An inventory control chart is a simple visual tool that helps you keep track of how much stock you have, when you need to reorder, and how much you should order. Think of it like a traffic light for your stock: green means you’re fine, yellow warns you to get ready, and red tells you to act fast. 🚦
Key Elements of the Chart
- Current Stock Level (Q) – the amount of goods you have on hand.
- Reorder Point (R) – the level at which you should place a new order.
- Order Quantity (OQ) – how many units you order each time.
- Lead Time (L) – how long it takes for the new stock to arrive.
How to Interpret the Chart
- Green Zone (Q > R) – You’re safe. No need to reorder yet.
- Yellow Zone (Q = R) – Prepare to reorder soon. It’s like the “almost empty” stage of a water bottle.
- Red Zone (Q < R) – You’re below the reorder point. Order immediately to avoid stock‑out.
Example: A Toy Store
Suppose a toy store sells 50 action figures per week. They keep a safety stock of 20 units and order 100 units whenever the stock falls to 30 units (the reorder point). The lead time is 2 weeks.
| Week | Sales | Stock Level (Q) | Reorder Point (R) | Action |
|---|---|---|---|---|
| 1 | 50 | 120 | 30 | Green – no order needed |
| 2 | 50 | 70 | 30 | Green – no order needed |
| 3 | 50 | 20 | 30 | Red – order 100 units now |
| 4 | 50 | 70 | 30 | Green – no order needed |
Mathematical Insight
The reorder point can be calculated as: $$ R = d \times L + SS $$ where $d$ is the average demand per period, $L$ is the lead time, and $SS$ is the safety stock. This formula ensures you never run out of stock during the waiting period for new goods. 📐
Quick Practice
- Imagine a bakery that sells 200 loaves per day. If the lead time for flour is 3 days and they keep a safety stock of 50 loaves, what is the reorder point?
- Draw a simple inventory control chart for a bookstore that sells 30 books per week, orders 120 books when stock falls to 40, and has a 1‑week lead time.
- Explain why the reorder point is higher when the lead time increases.
Take‑away Summary
- An inventory control chart is a quick visual check of stock levels. - The reorder point (R) tells you when to place a new order. - Keep a safety stock to cover unexpected demand or delays. - Use the formula $R = d \times L + SS$ to calculate the reorder point. - Regularly update the chart to avoid stock‑outs or over‑stocking.
Revision
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