Friday, March 29, 2013

How Much Does It Cost To Make Cookies?

“Buyers know what goods cost.” Some version of that assumption comes up in the very first weeks of just about every introductory econ course. It becomes one of the few assumptions that we make to build the model of consumer demand. But every once in a while, life gets in the way and asks “Is that something you really can assume?”

I had to test that assumption recently. I just moved and after unpacking, I was in the mood to make dessert for myself. Of course, I hadn't brought many kitchen supplies with me, so that quickly posed a problem. To make cookies, I needed to buy some wooden spoons, measuring cups, and a cooling rack. None of those are hard items to find, and I happened to live just minutes away from a shopping center that had a regional grocery store, a Wal-Mart, a Target, and a regional department store. I knew that all four stores should have what I want, so the question of where to go really came down to where it would cost the least. And that’s when I realized that one of the most basic assumptions of microeconomics didn't hold true. I didn't know which store would be the cheapest, or even what the prices of the goods should be!

I had some free time on a Saturday and a strong enough curiosity that I wanted to sample prices from each store. Here’s what I found:

(Dollars per spoon)
(Dollars per rack)
(Dollars per cup)
GROCERY STORE $1.50 $4.50 $1.22
WAL-MART $2.97 $2.99 $1.32
TARGET $2.03 $3.67 $4.97
DEPARTMENT STORE $12.00 $7.00 $7.50

I was also shocked by the spread in prices. While I did expect to see some markup at higher-end stores, the range was wider than I expected. I was also surprised that there wasn't one store that had the cheapest prices, across-the-board, for all the goods.

When economists create models, the goal is to make a few assumptions about the world to describe the “typical” human response and show how that response leads to a “general” outcome. My behavior in this case is not what economists would call “typical.” (My friends might even call it weird!) But even for the typical consumer, are the assumptions of the supply and demand model always appropriate?

In a lot of cases, the classic supply and demand model does gives accurate results, but sometimes the assumption that consumers know the distribution of prices isn't appropriate. In those cases, it’s important to understand how behavior will change if an assumption is violated. The classic model does not involve consumers looking for prices, they just know them. As economists, we often say we are assuming “complete information.” When consumers don’t have complete information the market price typically doesn't match the equilibrium price the model predicts. Most of the time the market will be inefficient (contrary to what the model suggests) and both producer and consumer surplus will be lost.

Throughout economics, every conclusion that we draw from a model depends on the assumptions that are used to build that model. Whenever I learn about a new model, I always list the assumptions made and focus on how the results change if the assumption would be removed. Understanding the relationship between assumptions and results is the critical step to applying what we learn from theory and using it to understand what happens in the real world.


1. When I was getting my information I found that stores rarely carry the exact same goods. (Even if they are the same brand, the packaging might be different. It’s why I calculated my information in per unit prices.) Since I was able to find the goods in multiple locations, but they were not identical, which market structure is the most appropriate to describe kitchen supplies: Monopoly, Oligopoly, Monopolistic Competition, or Perfect competition? Why?

2. While my shopping behavior was a bit different than most people for kitchen supplies, people do “search” when they buy certain goods. Name some items where the supply and demand model isn't as appropriate as a consumer search model would be. Why is it more appropriate to think about consumers searching for these goods?

3. An important part of search theory talks about the cost of searching. Suppose I didn't live near a shopping center and the stores were all 20 minute drives apart. How do you think that distance (and the opportunity cost associated with traveling between them) would change my behavior when I search? How would it change the pricing behavior of the stores?

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Wednesday, March 06, 2013

Assumption Corruption

A famous economics joke says it best: A physicist, a chemist and an economist are stranded on an island, with nothing to eat. A can of soup washes ashore. The physicist says, "Let’s smash the can open with a rock." The chemist says, "Let’s build a fire and heat the can first." The economist says, "Let’s assume that we have a can-opener..."

Though the above example is meant to be extreme, all economic models are based on underlying assumptions. They may pertain to variety of things such as the market structure, pricing mechanism, location, time lags, frictions, mathematical properties, etc., and they are used to simplify economic relationships.  An important assumption that drives many popular economic models is that of perfect information. In particular, models rely on the assumption that the prices of all goods and services are known.

As I was recently perusing the latest status updates of my Facebook friends, I noticed the following post: “How much do you pay a 14-year-old high school freshman as a mother’s helper a few days a week?”

As an economist, one might be inclined to quickly answer “whatever the market price is for that service!” However, it is clear from this post that that information is not actually known to the buyer.

While the internet has clearly helped to alleviate this information gap, it can still take time to gather all the relevant information necessary to make an informed decision. Sometimes, the cost of obtaining this information becomes so high that consumers decide not to research at all. For example, if you want to buy a hair dryer, a quick internet search may result in the same model offered at different stores for different prices, so you still wouldn’t know the relevant price for your needs without more digging.  

Even though the famous supply and demand model does not completely reflect the real world (since it assumes, among other things, that prices are known), this is not meant to imply that economic models are worthless. It would be impossible to model every detail of the “real world”; rather, it's important to make sure the assumptions are appropriate for what you are trying to analyze. For example, if you’re trying to model the effects of an increase in fuel price on consumers’ demand for SUVs, assuming perfect information for prices does not invalidate the results that it will decrease the demand for this good; it merely simplifies the model into something tractable. But in general, you’ll want to ask yourself the following questions when you examine an economic model: Are the assumptions of this model reasonable? Would changing the assumptions affect the result in a drastic way?

In short, be careful not to become a victim of assumption corruption, but don’t let fear of assumptions keep you away from using models at all either!

Discussion Questions

1. Another popular assumption is that agents act rationally and are utility-maximizers. How can this assumption still be valid in the presence of people volunteering their time or donating money?

2. Consider some other assumptions for the supply and demand model, such as price-taking behavior or the competitive hypothesis. How would relaxing those assumptions change the results of the model, if at all?

3. Why do we study economic models that don’t perfectly match our experience in the real world?

4. What are some other markets where the assumption of perfect information does not hold true in the real world?

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