Opportunity cost

   Economics is a discipline addresses on the issue of choice, and choice is associated with cost. If the choice brings change subsequently then we say such choice involves opportunity cost. Here comes the question for the present article: What is opportunity cost? Basically speaking, the highest-valued alternative forgone as a result of trade-offs is the opportunity cost of what is chosen. In other words, trade-offs involve a cost, which is what we call opportunity cost in economics.

   經濟學是一門著墨於選擇議題的學科，而選擇則與成本有關。若選擇會接著帶來改變，則我們說此種選擇包括了機會成本。這邊產生了本文的問題：甚麼是機會成本呢？基本上來說，我們所做的選擇之機會成本，乃是在進行權衡後所捨棄的選項之最高價值。換言之，權衡取捨包括了成本，而在經濟學中我們稱之為機會成本。

   Opportunity cost can be explained by the PPF (production possibilities frontier). PPF separates the goods and services that are attainable from those that are unattainable. Production of goods and services is possible at any point inside the attainable area and on the frontier. Production is not possible at any point outside the frontier.

   機會成本可由PPF（生產可能性曲線）來解釋。PPF將商品與服務分為可取得與不可取得的。在PPF曲線內部與線上任何點所進行的生產都是有可能做到的（可取得），而在線外的任何點則是無法做到的（不可取得）。

Example: The production of handbags and clothes

   As illustrated in the example, if one brand does not produce any handbag in that season, the maximum quantity of in-season clothes they can produce is 30. As they produce more luxury handbags, the quantity of clothes it is able to produce decreases. The line connects points A, B, C, D, and E together is the PPF. The opportunity cost is the slope of this line. It is a ratio without unit. In this example, if the quantity of handbag increases from 2 to 3, say, move from point C to point D, means the quantity of clothes produced decreases by 24-18 = 6. As a consequence, the additional one handbag produced costs 6 clothes and it is its opportunity cost.

   如本文範例所示，若一品牌在該季不打算生產任何手提包，則當季衣服最大的生產數量為30件。當他們生產愈來愈多手提包時，能生產的衣服數量就降低了。連結A、B、C、D、E的線即為PPF。機會成本是此線的斜率，是沒有單位的比值。本範例中，若手提包生產量從2增加至3，也就是從C點移至D點，意謂著衣服生產的數量減少為24-18 = 6。因此，這一個新增的手提包生產會造成6件衣服的支出，此即手提包的機會成本。

   As revealed in this example, the shape of the PPF is concave, and the opportunity cost is increasing for both goods. Under the prerequisites that the scarcity of resources and other factors of production remain the same, such phenomena can be explained as the more goods (handbag or clothes) we attempt to produce, the additional resources we use to produce are less productive, the opportunity cost of one additional unit of that good is larger as a consequence.

   如範例中所示，PPF呈現凹曲線型，而此二商品的機會成本都是隨數量遞增的。在資源有其匱乏性、以及其他生產要素保持不變的前提下，此種現象可解釋為當我們試圖生產更多商品（手提包或衣服）時，所增加用於生產的資源的生產力會每況愈下，因此每增加一個單位的商品時，機會成本會更大。

   The concept of opportunity cost can be applied widely. Except for the production of goods and services, we can also take this concept into our daily lives as well, such as our buying behaviour in department store when there is a big sale but our budget is limited.

   機會成本的概念可以被廣泛應用。除了生產商品與服務之外，我們也可將此概念應用至我們的日常生活中，像是當百貨公司有大特價、但我們預算有限時的消費行為上。