Overview

• What you’ll learn: The concept of relevant costs and revenues, how to identify sunk costs and exclude them from decisions, the role of opportunity costs, and the five-step decision framework.
• Prerequisites: Module 7 (Budgeting & Variance Analysis)
• Estimated reading time: 17 minutes

Introduction

The Grand Historian records: The greatest skill of the war council is not what they consider, but what they refuse to consider. A general who dwells on the cost of battles already lost will lose the next battle too. In the counting house of decision analysis, this ancient wisdom takes the form of a single principle: only relevant costs and revenues matter.

Horngren (Chapter 11) defines relevant information as expected future costs and revenues that differ among alternative courses of action. Two conditions must both be met: the cost must be (1) future-oriented (not already incurred) and (2) differential (different between alternatives). Costs that fail either test are irrelevant — noise that clouds judgment.

The Five-Step Decision Framework

1. Identify the problem and uncertainties: What decision must be made? What are the unknowns?
2. Obtain information: Gather relevant data — future, differential costs and revenues for each alternative.
3. Make predictions about the future: Forecast the likely outcomes of each alternative.
4. Choose an alternative: Select the option that best achieves the organization’s objectives (usually profit maximization or cost minimization).
5. Implement, evaluate, and learn: Execute the decision, measure results against predictions, and refine the process.

Sunk Costs: The Ghosts of Decisions Past

太史公曰：The emperor who sends reinforcements to a doomed fortress because he has already invested ten thousand troops is committing the sunk cost fallacy. Those troops are gone. The only question is whether sending more will improve the outcome or merely increase the losses.

A sunk cost is a past cost that cannot be changed regardless of which future action is taken. Examples:

• The purchase price of equipment already bought — whether you use it, sell it, or scrap it, the purchase price is unchanged.
• R&D costs already spent on a product — whether you launch or cancel, those costs are history.
• Training costs for employees who have already been trained — regardless of future assignments.

Sunk costs are irrelevant to decisions. They should be excluded from the analysis entirely. Yet the emotional pull of sunk costs is powerful — humans naturally seek to “justify” past investments, even when doing so leads to worse future outcomes.

Opportunity Costs: The Road Not Taken

An opportunity cost is the contribution to income forgone by not using a resource in its next-best alternative use. Opportunity costs are relevant but never appear in accounting records — they are the phantom army of decision analysis.

Example: A company owns a building it could rent for $50,000/year. If it uses the building for its own operations, the opportunity cost is $50,000 — the rent it sacrifices. This cost should be included in any analysis comparing self-use vs. alternative deployments.

Quantitative vs. Qualitative Factors

Not all relevant factors have dollar signs. Qualitative factors include:

• Employee morale: Outsourcing may save money but devastate workforce loyalty.
• Customer relationships: Dropping a product line may free capacity but alienate loyal customers.
• Quality and reputation: Cheaper materials may reduce costs but erode brand value.
• Legal and ethical considerations: The most profitable option may not be the most responsible.

The wise decision-maker quantifies what can be quantified, acknowledges what cannot, and weighs both.

Common Decision Traps

• Unitized fixed costs: Treating allocated fixed costs as variable when they are not. Dropping a product line does not eliminate the allocated corporate overhead — it merely redistributes it to remaining products.
• Book value of equipment: The book value of old equipment is a sunk cost, irrelevant to replacement decisions. The relevant factors are the disposal value of the old equipment, the cost of the new equipment, and the future operating cost differences.
• Ignoring opportunity costs: Failing to account for what resources could earn in their best alternative use.

Key Takeaways

• Relevant costs and revenues are future-oriented and differential between alternatives.
• Sunk costs are irrelevant — never let past expenditures drive future decisions.
• Opportunity costs are relevant but invisible in accounting records — include them explicitly.
• Qualitative factors must be weighed alongside quantitative analysis.
• The five-step decision framework provides structured discipline for managerial decisions.

What’s Next

In Lesson 2, we apply the relevant-cost framework to one of the most consequential decisions a company faces: make-or-buy and outsourcing.