ACCA PM: The Sunk Cost Candidates Still Include in Relevant Costing Answers

Relevant costing questions in ACCA Performance Management (PM) are straightforward once the logic clicks: strip a decision down to the cash flows that will actually change because of it. The trap is that most of the numbers a question hands you were never meant to be used, and one of them, the sunk cost, keeps making its way into candidates' answers anyway.

This is one of the most consistent ways candidates lose marks in short-term decision-making questions — not because the concept is misunderstood in the abstract, but because it is misapplied the moment a question dresses a sunk cost up as something that looks decision-relevant. If you are working through this material as part of your ACCA PM studies, this is worth reading before your next relevant costing question, not after.

What "relevant" actually means in relevant costing

A relevant cost or revenue is one that will change as a direct result of the decision being made. To qualify, a cash flow must satisfy three conditions at once: it must be a future cash flow, it must be incremental (arising specifically because of this decision, not regardless of it), and it must actually be a cash flow — non-cash items such as a depreciation charge are excluded, since depreciation is an accounting allocation, not money moving. A cost that fails even one of these three tests is irrelevant and should be left out of the analysis, no matter how prominently the question presents it.

Why sunk costs have to be excluded

A sunk cost is money already spent, or a cost the business is already irrevocably committed to, before the decision is even made. Because it belongs to the past, it fails the "future" test outright — and whatever choice is made now cannot change it. The cash is gone whether the project goes ahead, is scaled back, or is abandoned entirely. Including it double-counts a cost with no bearing on the decision, and can flip a correctly profitable decision into one that wrongly looks unviable.

The classic exam setup: a question states that £15,000 was spent last year investigating a new product line's viability, then asks whether the line should now be launched. That £15,000 is sunk. It was spent regardless of today's decision and cannot be recovered either way, so it plays no part in the calculation — even though it is often the single largest, most attention-grabbing figure in the question.

The three traps that travel with the sunk cost error

Once you are alert to sunk costs, three closely related traps tend to catch the same candidates, because they all involve a cost that looks like it belongs in the calculation but fails one of the three relevance tests:

• Committed (unavoidable) fixed costs. A fixed cost the business is already contractually locked into — an equipment lease with two years left to run, for example — will be paid regardless of the decision, so it is not incremental and must be excluded, just like a sunk cost, even though it is a genuinely future cash flow rather than a past one.
• Historic purchase cost of materials or equipment already owned. What was originally paid for an item already in inventory or on the balance sheet is sunk. What matters instead is its current relevant value: replacement cost if it is regularly used and would need replacing, or net realisable (resale) value if it is obsolete and would otherwise be scrapped or sold.
• Opportunity cost of a resource with an alternative use. This trap runs the other way: candidates correctly strip out a sunk cost, then wrongly assume the resource is now "free" to use. If an existing machine, material, or staff member has an alternative use — hired out, sold, or redeployed elsewhere — the benefit given up by using it here instead is a real, relevant opportunity cost that must be added into the analysis.

A quick test before you commit to an answer

Before including any figure in a relevant costing calculation, ask one question: will this cash flow actually change, one way or the other, depending on which option is chosen? If the amount stays exactly the same regardless of the decision — because it has already been spent, or the business is already contractually committed to it — it fails the test and comes out of the calculation, however prominent it looks. If a resource is being used at no apparent extra cash cost but has a genuine alternative use elsewhere, ask what is being given up by using it here instead, and bring that opportunity cost into the numbers. Running every figure through this filter before building the final calculation is what catches sunk costs and unavoidable fixed costs before they reach an answer.

It is also worth remembering that relevant costing is distinct from the absorption-versus-marginal costing distinction tested elsewhere in the syllabus: full absorption costs allocate historic fixed overheads across units for reporting purposes, while a relevant costing decision cares only about which future cash flows change. Mixing the two frameworks up is another route back into the same sunk-cost trap.

Frequently asked questions

What makes a cost relevant to a decision?

A relevant cost must be a future cash flow that arises specifically as a result of the decision being made (incremental) and must represent an actual movement of cash rather than a non-cash entry such as depreciation. A cost that fails any one of these conditions is not relevant and should be excluded.

Why can't a sunk cost be recovered by choosing a particular option?

A sunk cost has already been paid, or the business is already irrevocably committed to it, before the decision is made. Because it exists independently of which option is chosen, no decision made now can change or recover it, so it has no bearing on which option is financially better and must be left out.

Is opportunity cost the same thing as a sunk cost?

No — they are opposites in effect. A sunk cost is excluded because it cannot be changed by the decision. An opportunity cost is included because it represents a real future benefit given up when a resource with an alternative use is deployed on the option being evaluated, so it is very much relevant.

Sunk costs, unavoidable fixed costs, and opportunity costs form one connected idea in relevant costing: strip out what the decision cannot change, and bring in what it genuinely gives up. Practising this discipline alongside related short-term decision-making techniques, like those covered in ranking decisions under a bottleneck constraint, is what turns relevant costing from a source of lost marks into a reliable one. Learnsignal's ACCA PM course builds this technique step by step through exactly this kind of scenario-based question.