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

  1. Green Zone (Q > R) – You’re safe. No need to reorder yet.

  2. Yellow Zone (Q = R) – Prepare to reorder soon. It’s like the “almost empty” stage of a water bottle.

  3. 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.

WeekSalesStock Level (Q)Reorder Point (R)Action
15012030Green – no order needed
2507030Green – no order needed
3502030Red – order 100 units now
4507030Green – 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

  1. 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?

  2. 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.

  3. 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.