Opportunity cost
trade-off · cost of alternatives
The cost of a choice is the most valuable alternative given up to make it. Not the money spent — the thing forgone. On James Buchanan's account (1969) it is irreducibly subjective, exists only at the moment of decision, and is never observed, because the alternative never happens.
In practice
A retainer at your standard rate looks like revenue. Its cost is the month of product work it displaces, which never appears on any statement, is never invoiced against, and is the reason three good years can leave a practice exactly where it started.
The common mistake
Counting only what was paid. A decision that costs nothing in cash can be the most expensive one available, and it will not show up anywhere in the accounts. The adjacent error is confusing it with sunk cost: opportunity cost is forward-looking, sunk cost is already gone and is not a cost of anything.
Every choice is a refusal. The cost of what you did is what you would otherwise have done with the same time, money or attention — and because that alternative never occurs, the cost is never invoiced, never recorded, and almost never counted.
Where the idea comes from
Frédéric Bastiat (1850) set out the structure before the term existed, in 'What is Seen and What is Not Seen'Bastiat, F. (1850). 'Ce qu'on voit et ce qu'on ne voit pas.' The broken-window parable: the glazier's earnings are seen; the shoes the shopkeeper would otherwise have bought are not. Bastiat's claim is that the whole difference between a good and a bad economist is whether the unseen is included.: the good economist accounts for effects that do not happen, the bad one for the effects in front of him. David Green gave the English term its shape in 1894, and Friedrich von Wieser developed the doctrine of alternative cost — that the value of a resource is set by the best use it is withheld from, not by what it cost to obtain.
James Buchanan (1969) pushed it to its sharpest statement. Cost is not an objective magnitude sitting in the world to be measured; it is the chooser's own evaluation of the displaced alternative, it exists only at the moment of choice, and it is never realisedBuchanan, J. M. (1969). Cost and Choice: An Inquiry in Economic Theory. Markham. If cost is the value of the alternative not taken, then by construction it is never experienced and can never be measured after the fact — which is why decisions cannot be audited on cost in the way budgets can.. You never find out what the road not taken was worth. This is why the concept resists the accounting treatment everyone tries to give it.
The measurement problem, demonstrated
Paul Ferraro and Laura Taylor (2005) put a four-option opportunity-cost question to professional economists at an American Economic Association meeting. 21.6 per cent answered correctlyFerraro, P. J. & Taylor, L. O. (2005). 'Do Economists Recognize an Opportunity Cost When They See One? A Dismal Performance from the Dismal Science.' B.E. Journal of Economic Analysis & Policy 4(1). Random guessing over four options yields 25 per cent. Performance did not improve with seniority. — below chance. The point is not that economists are careless; it is that the calculation is genuinely hard, because it requires pricing something that does not exist, and no amount of familiarity with the term makes that automatic.
What it is not
It is not sunk cost. Money and time already spent are gone under every available option, so they are not a cost of any of them; Arkes and Blumer (1985) documented how reliably people fail to drop them. Opportunity cost points forward and sunk cost points backward, and treating the second as the first is the most common single error in decision-making. It is also not accounting cost — a choice can cost nothing in cash and still be the most expensive one on the table, which is precisely the case the concept exists to catch.
The objections
The concept has a well-known operational weakness: taken literally, comparing against the best alternative requires enumerating the alternatives, and the set is unbounded. In practice the comparison is made against a small, arbitrary shortlist, and the answer depends on what happened to be on it — which makes the analysis feel rigorous while resting on an unexamined step. The Austrian response is that this is a fact about choice rather than a flaw in the concept.
The subjectivist reading has a sharper cost. If Buchanan is right that cost is private and unrealised, opportunity cost cannot function as an auditable management number, and any attempt to put it in a report has quietly changed the definition into something measurable and therefore into something else.
What it rules out
It rules out judging a decision by whether it worked. The relevant comparison is against the alternative, not against nothing, and profitable work can carry a loss. It rules out 'it didn't cost us anything' as a defence for anything that consumed time. And it rules out treating a full calendar as evidence of productivity, since a full calendar is a statement about what was refused.
It does not rule out taking the lesser option knowingly. A decision made with the cost in view is a different act from one made with the cost invisible, and the concept's whole function is to move the choice from the second category to the first.
Sources
Arkes, H. & Blumer, C. (1985). 'The Psychology of Sunk Cost.' Organizational Behavior and Human Decision Processes 35(1). · Bastiat, F. (1850). Ce qu'on voit et ce qu'on ne voit pas. · Buchanan, J. M. (1969). Cost and Choice. Markham. · Ferraro, P. J. & Taylor, L. O. (2005). 'Do Economists Recognize an Opportunity Cost When They See One?' B.E. Journal of Economic Analysis & Policy 4(1). · Green, D. I. (1894). 'Pain-Cost and Opportunity-Cost.' Quarterly Journal of Economics 8(2). · Wieser, F. von (1914). Theorie der gesellschaftlichen Wirtschaft.
Concept web
Open the full webQuestions
What is opportunity cost?
The value of the best alternative given up in making a choice. Because the alternative never happens, the cost is never recorded anywhere — Buchanan (1969) argued it is subjective, exists only at the moment of decision, and is never realised.
What is the difference between opportunity cost and sunk cost?
Opportunity cost is forward-looking: what you give up by choosing this. Sunk cost is backward-looking: what is already spent and unrecoverable under every option, and therefore not a cost of any of them. Treating sunk cost as a reason to continue is the error Arkes and Blumer documented in 1985.
Why is opportunity cost so hard to apply?
It requires pricing something that does not exist. Ferraro and Taylor (2005) asked professional economists a four-option opportunity-cost question and 21.6 per cent answered correctly — below the 25 per cent expected from guessing.
Can opportunity cost be measured?
Not reliably. If cost is the value of an alternative that never occurred, there is nothing to observe afterwards. Any figure that appears in a report has substituted a measurable proxy for the concept, which is useful but is no longer opportunity cost.