Opportunity Cost and Comparative Advantage
AdvancedOpportunity cost is the highest-valued option forgone, covering money and time. Whoever faces a lower opportunity cost has a comparative advantage; specialising accordingly and trading benefits both sides — even if one is better at everything.
- Opportunity cost = the highest-valued forgone option, never the sum of options
- Full opportunity cost = direct money spending + forgone earnings − income still earned
- Interest is the cost of obtaining resources earlier; borrowing and spending savings both carry it
文體小知識：經濟現象說明文——以數字計算講解機會成本與比較優勢，說明專門化和交易如何創造貿易得益。
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1Opportunity cost is the highest-valued option forgone when a choice is made — not the sum of all forgone options. It covers more than money: time and other non-monetary sacrifices count too. A S6 graduate who continues studying gives up the earnings of a full-time job; one who takes a job gives up the qualifications and future income of study. Value is subjective and differs across people, so the same choice can carry different opportunity costs for different people.
2Interest is the cost of obtaining resources earlier. Instalment plans usually cost more than the cash price, and the difference is the price of enjoying the good sooner; spending one's savings means forgoing interest income, which is also a cost. Economics reminds us to count the price of 'earlier' in opportunity cost — this applies to borrowing and to using one's own savings alike.
3Specialisation means an individual or region concentrates on producing a few goods or tasks, and exchange is its precondition: without trade, someone producing only one good could not obtain the rest. In a tea restaurant, cooks cook and servers serve — specialisation in daily life. It applies between cities too: Hong Kong focuses on financial and business services while mainland cities supply food and manufactures, and the two trade to mutual benefit.
4The principle of comparative advantage explains the source of gains from trade: even if one party produces more of everything — an absolute advantage — both sides gain as long as each specialises where its opportunity cost is lower (its comparative advantage) and trades at a ratio between their opportunity costs. Trade is therefore not zero-sum: comparative advantage is relative, everyone has some line of production with lower opportunity cost, and specialisation raises total output.
1Summary 重點摘要
Opportunity cost is the highest-valued option forgone, covering money and time. Whoever faces a lower opportunity cost has a comparative advantage; specialising accordingly and trading benefits both sides — even if one is better at everything.
2Vocabulary cards Vocabulary
The highest-valued option forgone when a choice is made, covering monetary and non-monetary sacrifices.
Among the options given up, the one valued most highly by the decision-maker; it defines opportunity cost.
Direct spending in money terms — only part of the full opportunity cost.
The cost of obtaining resources earlier: borrowing incurs interest, and spending savings forgoes interest income.
Concentrating on producing a few goods or tasks to raise efficiency.
Swapping goods or services; exchange is the precondition of specialisation.
The party that produces more output with the same resources (or the same output with fewer resources).
The party with a lower opportunity cost of producing a good; one can hold it without any absolute advantage.
After specialising by comparative advantage and trading, both sides obtain goods more cheaply than making them, and total output rises.
The rate at which two goods exchange; for both sides to gain, the rate must lie between their opportunity costs.
3Step by step Steps
- Step 1: The full definition of opportunity costOpportunity cost is the highest-valued option forgone when a choice is made — take the single highest one, never add up all options. Value is judged subjectively by the decision-maker, and behaviour usually reveals the ranking. For example, S6 graduate Ah Kin has three options: a full-time job paying $14,000 a month, an associate degree, or studying abroad. He takes the associate degree, so his opportunity cost is whichever of the job and overseas study he values more — not the two added together.
- Step 2: Three steps to compute opportunity costTo compute opportunity cost: first list all forgone options; then compare their value to the decision-maker; finally take the highest-valued one. For example, on Saturday afternoon Ah Kei could earn $600 in a fast-food job, join an inter-school competition, or revise at home. She joins the competition, and the option she most wanted among those forgone was the $600 job — so the opportunity cost of competing is $600; the value of revising at home is not counted.
- Step 3: Money cost is not the full costMoney spending is only part of opportunity cost; add forgone earnings and time, and subtract any money still earned during the chosen option. For example, Ah Ming takes a two-year course with annual fees of $30,000, giving up a $14,000-a-month job while still able to earn $3,000 a month part-time. Over two years, the full opportunity cost is fees of $60,000, plus forgone earnings of $336,000, minus part-time income of $72,000 — a total of $324,000.
- Step 4: Interest is the cost of having resources earlierThe sooner you obtain the same good, the higher the cost. Instalment prices usually exceed cash prices, and the difference is the price of enjoying the good earlier; even without borrowing, spending savings forgoes interest income, which is also a cost. For example, a phone costs $5,000 in cash but $5,400 over twelve instalments — the extra $400 is the price of using the phone sooner; paying from savings also forgoes deposit interest, part of that choice's opportunity cost.
- Step 5: Specialisation and exchange go hand in handSpecialisation — concentrating resources on a few goods or tasks — raises proficiency and efficiency, but it depends on exchange, because specialists need to obtain other goods. For example, in a tea restaurant the drinks counter makes beverages, the kitchen fries noodles and the floor staff serve and take payment; division of tasks boosts efficiency, and wages exchange their services. Between regions likewise: Hong Kong specialises in services and imports food and manufactures from elsewhere.
- Step 6: Absolute advantage versus comparative advantageAbsolute advantage is about output: with the same resources, whoever produces more has it. Comparative advantage is about opportunity cost: whoever gives up less of the other good to make one unit has it. For example, Ah Ming can make 10 cups of milk tea or 20 toasts per hour; Ah Kei can make 6 cups or 18 toasts. Ah Ming is better at both — absolute advantage. But one cup of milk tea costs Ah Ming 2 toasts and Ah Kei 3, so Ah Ming has the comparative advantage in milk tea.
- Step 7: Trade along comparative advantage benefits bothContinuing the example: one toast costs Ah Ming half a cup of milk tea but Ah Kei only about one-third, so Ah Kei has the comparative advantage in toasts. If each specialises accordingly — Ah Ming makes milk tea only, Ah Kei toasts only — total output exceeds what either could achieve alone. As long as one cup of milk tea trades for between 2 and 3 toasts, the ratio lies between their opportunity costs, and both obtain the other good more cheaply than making it: trade benefits both.
5Worked example Example
Calculation (two parts): 1. After S6, Ah Wai enrols in a two-year associate degree with annual fees of $30,000; otherwise he could work full time at $14,000 a month, and while studying he can still earn $3,000 a month part-time. Find the opportunity cost of the two-year degree. 2. Ah Ming can make 10 cups of milk tea or 20 toasts per hour; Ah Kei can make 6 cups or 18 toasts. Who has the comparative advantage in milk tea? Within what range must the exchange ratio lie for both to gain from specialising?
- Part 1 set-up: opportunity cost includes forgone wages and fees, but subtract part-time income still earned while studying
- Compute: forgone earnings 14,000 × 24 = 336,000; fees 30,000 × 2 = 60,000; part-time income 3,000 × 24 = 72,000; opportunity cost = 336,000 + 60,000 − 72,000 = $324,000
- Part 2 compute opportunity costs: one cup of milk tea costs Ah Ming 20 ÷ 10 = 2 toasts and Ah Kei 18 ÷ 6 = 3 toasts, so Ah Ming has the comparative advantage in milk tea
- One toast costs Ah Ming 10 ÷ 20 = 0.5 cup and Ah Kei 6 ÷ 18 ≈ 0.33 cup, so Ah Kei has the comparative advantage in toasts; specialisation raises their total output
- Conclusion: the exchange ratio must lie between their opportunity costs — 1 cup of milk tea for 2 to 3 toasts — for both to gain
- Answer: 1. The opportunity cost of Ah Wai's two-year degree = forgone earnings $336,000 + fees $60,000 − part-time income $72,000 = $324,000. 2. One cup of milk tea costs Ah Ming 2 toasts and Ah Kei 3 toasts, so Ah Ming holds the comparative advantage in milk tea and Ah Kei in toasts. After specialising accordingly, any exchange ratio between 1 cup of milk tea for 2 and 3 toasts lets each side obtain the other good more cheaply than making it — trade benefits both.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the definition of opportunity cost?
Q2Three options are worth $500, $700 and $300. What is the opportunity cost of choosing the $500 option?
Q3A course costs $20,000 in fees and requires giving up $150,000 of earnings. What is the opportunity cost?
Q4Ah Ming's opportunity cost of one cup of milk tea is 2 toasts, Ah Kei's is 3. Who has the comparative advantage?
Q5Within what range must the terms of trade lie for both sides to gain?
8Recap Recap
- Opportunity cost = the highest-valued forgone option, never the sum of options
- Full opportunity cost = direct money spending + forgone earnings − income still earned
- Interest is the cost of obtaining resources earlier; borrowing and spending savings both carry it
- Comparative advantage is about lower opportunity cost — not the same as absolute advantage
- Specialising by comparative advantage and trading between the two opportunity costs benefits both sides