理性人考虑边际量

Rational people think at the margin

Economists use the term marginal changes to describe small incremental adjustments to an existing plan of action.In many situations,people make the best decisions by thinking at the margin．

Suppose,for instance,that you asked a friend for advice about how many years to stay in school.If he were to compare for you the lifestyle of a person with a Ph.D.to that of a grade school dropout,you might complain that this comparison is not helpful for your decision.You have some education already and most likely are deciding whether to spend an extra year or two in school.To make this decision,you need to know the additional benefits that an extra year in school would offer (higher wages throughout life and the sheer joy of learning) and the additional costs that you would incur (tuition and the forgone wages while you're in school) .By comparing these marginal benefits and marginal costs,you can evaluate whether the extra year is worthwhile．

As another example,consider an airline deciding how much to charge passengers who fly standby.Suppose that flying a 200-seat plane across the country costs the airline $100,000.In this case,the average cost of each seat is $100,000/200,which is $500.One might be tempted to conclude that the airline should never sell a ticket for less than $500.In fact,however,the airline can raise its profits by thinking at the margin.Imagine that a plane is about to take off with ten empty seats,and a standby passenger is waiting at the gate willing to pay $ 300 for a seat.Should the airline sell it to him? Of course it should.If the plane has empty seats,the cost of adding one more passenger is minuscule.Although the average cost of flying a passenger is $500,the marginal cost is merely the cost of the bag of peanuts and a can of soda that the extra passenger will consume.As long as the standby passenger pays more than the marginal cost,selling him a ticket is profitable．

As these examples show,individuals and firms can make better decisions by thinking at the margin.A rational decision maker takes an action if and only if the marginal benefit of the action exceeds the marginal cost．

理性人考虑边际量

生活中的许多决策涉及对现有行动计划进行微小的增量调整,经济学家把这些调整称为边际变动。在许多情况下,人们可以通过考虑边际量来作出最优决策。

例如,假设你请教一位朋友,你应该在学校上多少年学。如果他给你用一个拥有博士学位的人的生活方式与一个完全没有上完小学的人进行比较,你会抱怨这种比较无助于你的决策。你已经受过某种程度的教育,并要决定是否再多上一两年学。为了作出这种决策,你需要知道,多上一年学所带来的额外收益和所花费的额外成本。通过比较这种边际收益与边际成本,你就可以计算出多上一年学是否值得。

再举一个考虑边际量如何有助于作出决策的例子。考虑一下一个航空公司决定对等退余票的乘客收取多高的价格的例子,假设一架200个座位的飞机横越国内飞行一次,航空公司的成本是10万美元。在这种情况下,每个座位的平均成本是10万美元除以200,即500美元。有人会得出结论: 航空公司的票价绝不应该低于500美元。但航空公司可以通过考虑边际量而增加利润。假设一架飞机即将起飞时仍有10个空位。而在登机口等退余票的乘客愿意支付300美元买一张票。那么航空公司应该卖给他票吗? 当然应该。如果飞机有空位,多增加一位乘客的成本是微乎其微的。虽然一位乘客飞行的平均成本是500美元,但边际成本仅仅是这位额外的乘客将消费的一包花生和一罐汽水的成本而已。只要等退余票的乘客所支付的钱大于边际成本,卖给他机票就是有利可图的。

正如这些例子说明的,个人和企业通过考虑边际量将会作出更好的决策。只有一种行动的边际收益大于边际成本时,一个理性决策者才会采取这项行动。