Why this is mistaken
The mistake sees familiar totals or averages and uses them even though the decision concerns one change. That can reverse a decision when early units and later units have different benefits or costs.
Repair the reasoning with two columns: current baseline and baseline plus the specified increment. Subtract line by line. Unchanged amounts reconcile but do not enter the marginal comparison. Label contingent effects and opportunity costs rather than hiding them in an average.
The diagnosis is conceptual, not merely arithmetic. Someone who computes the difference correctly by accident should still explain why total, average, marginal, and sunk measures answer different questions.
Where to watch
When this mistake may appear
- A prompt asks whether to add one unit, review, employee, customer, or production run.
- Average cost and incremental cost move differently.
Check your work
Your work may contain this mistake if:
- Uses total historical spending as the cost of one more unit.
- Uses average benefit without estimating the change from the current baseline.
- Includes amounts unchanged by either alternative.