Overview

• What you’ll learn: The Economic Order Quantity (EOQ) model, safety stock and reorder points, just-in-time (JIT) production systems, backflush costing, and the trade-offs between carrying costs and stockout costs.
• Prerequisites: Lesson 2 — Time as a Competitive Tool
• Estimated reading time: 17 minutes

Introduction

The Grand Historian records: The general who stockpiles too many provisions drains the treasury with warehousing costs and watches grain rot in storage. The general who stockpiles too few watches his army starve when supply lines are disrupted. Inventory management is the ancient art of balance — enough to sustain operations, not so much as to bankrupt the enterprise.

Horngren (Chapter 20) presents inventory management as a continuum from traditional EOQ-based systems to modern JIT philosophy, each with distinct cost trade-offs and operational implications.

Inventory Costs

The EOQ Model

The Economic Order Quantity minimizes the total of ordering and carrying costs:

EOQ = sqrt(2 × D × S / H)

Where: D = annual demand, S = ordering cost per order, H = carrying cost per unit per year.

Assumptions (and their violations in reality): constant demand rate, constant lead time, instantaneous receipt of orders, no quantity discounts. Despite these simplifications, EOQ provides a useful baseline.

Reorder Point and Safety Stock

Reorder Point = (Demand per day × Lead time in days) + Safety Stock

Safety stock is the buffer against demand and supply uncertainty. Higher safety stock reduces stockout risk but increases carrying costs. The optimal safety stock balances the expected cost of stockouts against the carrying cost of additional inventory.

Just-in-Time (JIT) Production

JIT flips the inventory paradigm: instead of producing to build inventory (push system), produce only what is needed, when it is needed, in the quantity needed (pull system).

Core Principles

• Demand-pull production: Production is triggered by actual customer orders, not forecasts.
• Elimination of waste: Any activity that does not add value is waste — including excess inventory, which JIT considers the “root of all evil” because it hides problems.
• Continuous improvement (Kaizen): Small, incremental improvements every day.
• Total quality: Defects stop the line — you cannot pass defective work downstream.
• Supplier partnerships: Frequent, small deliveries from reliable suppliers located nearby.

JIT and Inventory as a Problem-Hider

太史公曰：High water in the river hides the rocks beneath. When the water level drops, the rocks are revealed and the boat runs aground. Inventory is the water; operational problems are the rocks. JIT deliberately lowers the water level to expose and fix the rocks — machine breakdowns, quality defects, unreliable suppliers, long setup times.

Backflush Costing

Traditional cost accounting tracks costs at each production stage (materials → WIP → finished goods → COGS). JIT systems with minimal inventory can simplify this with backflush costing, which delays cost recognition until goods are completed or sold, then “flushes” costs backward through the system.

Backflush costing works when: WIP is negligible, cycle times are short, and production closely matches sales. It fails when significant WIP exists or when detailed work-in-process tracking is needed.

Key Takeaways

• Inventory costs include carrying, ordering, stockout, and quality costs — they pull in opposite directions.
• EOQ minimizes ordering + carrying costs; safety stock buffers against uncertainty.
• JIT is a demand-pull system that eliminates waste by producing only what is needed when needed.
• Inventory hides operational problems — JIT exposes them for resolution.
• Backflush costing simplifies cost tracking for lean environments with minimal WIP.

What’s Next

In Lesson 4, we turn from operational management to long-term investment — capital budgeting methods for evaluating major expenditure decisions.