Thursday, July 26, 2012

Preferences Are a Ranking, Not a Measurement Using Numbers - Robert P. Murphy

Because preferences are tied to a person’s exchanges, the preferences can only reveal a ranking of goals. When Mary chooses vanilla over chocolate ice cream, this purposeful action only indicates that she prefers vanilla. We can’t determine “how much” Mary prefers vanilla over chocolate; indeed, that statement doesn’t even make sense in terms of strict economic logic.
In everyday conversation, we all know what it means to say that “Mary really prefers vanilla over chocolate but her sister Jane only slightly prefers vanilla to chocolate.” But it’s important for you to see that this type of talk makes no sense in terms of the preferences that we use in economic reasoning.
After all, what does it really mean—from the standpoint of pure economic logic—to say that Mary has a preference for vanilla over chocolate? All it means is that, faced with a choice between the two flavors, Mary would pick vanilla. But that is the same thing we can say about her sister Jane, whose friends would testify that she has only a “slight” preference for vanilla. Jane too, when faced with a choice, would pick vanilla over chocolate. So in terms of logical deductions that we can make based on a person’s purposeful actions, all we can say as economists is that both girls exhibit a preference for vanilla over chocolate.
We can take this train of thought further to drive home the lesson. Even if Jane announces, “I just barely prefer vanilla to chocolate!” that wouldn’t give an economist the ability to conclude that her preference for vanilla is “less intense” than Mary’s. No, it would merely allow the economist to conclude that Jane preferred to yell that particular sentence, versus yelling something else or keeping her mouth shut. Remember, we are using the notion of a person’s subjective preferences to explain the concrete actions that the person takes. If someone utters a statement, that informs economists about the person’s preferences all right, but only because the utterance itself is a purposeful action!3
To help you remember the points of this lesson, consider the analogy of friendship. For example, Sally might have three friends, and so we could say that in her mind she holds feelings of friendship for each of them. We can push it further and ask Sally to rank her friends. She might say that Bill is her best friend, that Mary is her second-best friend, and that Joe is her third-best friend. Such talk is perfectly meaningful.
But what if we then asked Sally how much better a friend Bill was than Mary? Now things start to sound a little strange. And if we asked her, “Does Bill possess at least 30% more friendship than Joe?” we would have entered the realm of the absurd. The moral of this story is that it makes sense to rank friends, but even so there’s still no such thing as an objective “unit of friendship” behind the scenes, driving our ranking.
The same is true with preferences in general, at least as we use them in economics. As you will learn in upcoming lessons, to understand and describe exchanges, we need to assume that people have a ranking of goals or ends. People take actions to satisfy their most important preferences, or to achieve their highest goals. We do not have to say that people have a mathematical “utility function” that they seek to maximize, even though such talk is commonplace in other economics textbooks. This alternate approach is only useful in coming up with specific answers to contrived numerical problems; it doesn’t actually shed more understanding on the process of exchange. In fact, the use of mathematical utility functions is very harmful when learning basic economic principles, because it often causes the student to forget where the notion of preference comes from in the first place.

An Alternate View
Even professional economists do not always heed the principle that preferences are a ranking, not a measurement. For example, economists often use the term utility to describe how much pleasure or satisfaction a person gets from a particular situation. Therefore they might describe our scenario by saying, “Mary chose vanilla ice cream because it gave her more utility than the chocolate ice cream would have given her.”
So far, so good. But then many economics textbooks push it further and start assigning numbers to measure how much utility, so that (say) Mary gets “55 utils” from vanilla but only “34 utils” from chocolate, and so in order to “maximize utility” she obviously chooses the vanilla. If you are taking a Ph.D-level class, the textbook will explain that “utils” don’t really exist, the way “kilograms” are an objective unit of weight and “meters” are an objective unit of height. Instead, the Ph.D.-level textbook will explain, economists can use mathematical utility functions just as a convenient shortcut to describing preference rankings. So when the function assigns “55 utils” to a bowl of vanilla ice cream but only “34 utils” to the chocolate, all that really means is that Mary would choose the former over the latter. The utility function could just as well have assigned “18.7 utils” to the vanilla and “2.3 utils” to the chocolate; the important thing is that Mary acts “as if” she is maximizing this arbitrary mathematical function.
In this book, we will not be using the confusing terminology of “utils,” and we won’t be performing calculus on “utility functions” the way other economics textbooks do. These practices, though common, are dangerous because they can mislead you into thinking that we are measuring the amount of psychic satisfaction an individual derives from particular actions.
It may be that one day neuroscientists come up with an objective way to quantify various degrees of happiness, such that they can coherently talk about Mary being “three times more satisfied” than Bill. But even if this happens, our point here remains the same: In the field of economics, such talk is meaningless. In economics, we use terms like “preferences” as a way to explain or describe the purposeful actions of individuals. When someone chooses one thing over another, all we can conclude is that the person preferred the chosen item over the discarded item. Psychologists or neuroscientists (or even common sense) might shed more light on the event, but economic logic per se can go no further. The economist isn’t claiming to have all the answers; far from it! The economist is actually being humble here by admitting the limits of what economic reasoning can say about a given event. In Lesson 6, we will see how subjective preference rankings interact to yield objective market prices. At that time, you will understand better why we are stressing these points in this lesson.

Lessons for the Young Economist

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