Private and Common Values

students, imagine walking into an auction where everyone is trying to guess how valuable an item really is. Some people know a lot, some know a little, and sometimes the item’s true value is the same for everyone but still hidden. 🎯 This lesson explains private values and common values, two important ways economists think about uncertainty in auctions and other market settings.

By the end of this lesson, you will be able to:

Distinguish between private values and common values.
Explain why information matters in auctions.
Identify the value structure in simple examples.

These ideas show up in real life all the time: bidding on a used phone, buying land, bidding for oil drilling rights, or even competing for advertising space online. The key question is simple: Who knows what, and how does that affect the price? 💡

Private Values: When Each Person Has Their Own Value

In a private value setting, each bidder has their own personal value for the item, and that value does not depend on what other bidders think. In other words, if you value a concert ticket at $80, that is your own number based on your preferences, plans, and budget. Someone else might value the same ticket at $120 because they are a bigger fan or have been waiting longer.

A simple way to think about private values is this:

Your value is yours alone.
Other bidders’ information does not change your value.
The item is worth different amounts to different people because they want it differently.

For example, students, imagine three students bidding on a used textbook:

Student A needs it for a class next semester and values it at $45$.
Student B already has an older edition and values it at $20$.
Student C wants it only if the price is low and values it at $30$.

This is a private values auction because each student’s value comes from personal use. If Student A learns that Student B thinks the book is worth only $20$, Student A’s own value still stays $45$.

Private values are common when the good is mainly valuable because of personal preferences. Examples include:

a sweatshirt from your favorite team 🏈
a lunch item you personally enjoy
a seat at a concert
a used game console for your own use

In this setting, a bidder’s best estimate of what to pay is based mostly on their own value. The danger is overbidding and paying more than the item is worth to you.

Common Values: When the Item Has One True Value for Everyone

In a common value setting, the item has one underlying value for everyone, but no one knows it exactly at the time of bidding. Each bidder may have different information, but the true value is the same for all.

A good example is an oil lease. The oil in the ground has one market value, but the exact amount of oil is unknown. Different bidders may have different geological reports, but the oil field itself has only one true value.

You can think of common values like this:

The item has one actual worth.
Bidders are guessing that worth.
Different bidders may have different clues.
The best-informed bidder may have an advantage.

Suppose a company is bidding on land that might contain copper. One engineer estimates the land is worth $10$ million, another estimates $14$ million, and a third estimates $12$ million. The true value is not known yet, but if the land is really tied to the amount of copper underneath, then everyone is trying to estimate the same underlying value. That is a common value situation.

Common values appear in markets like:

oil and mineral rights ⛏️
government contracts where future profits are uncertain
financial assets whose future resale value is unclear
certain online ad auctions where the value depends on future clicks or sales

The big idea is that the item’s value is not just about what you personally want. It depends on an unknown fact about the world.

Why Information Matters in Auctions 📊

Information changes bidding behavior in both private and common value auctions, but it matters much more in common value settings.

In a private value auction, each bidder knows their own value. Extra information about other bidders usually helps you predict their bids, but it does not change your own value. If you are willing to pay up to $50$, then learning someone else values the item at $20$ does not change your personal limit.

In a common value auction, extra information can be extremely important because bidders are not just estimating the item’s value—they are estimating it with incomplete information. If one bidder has better information, others may lose if they rely too much on their own estimate.

This creates a famous problem called the winner’s curse. The winner’s curse happens when the bidder who wins is often the one who was most optimistic, meaning that bidder may have overestimated the true value. After winning, the bidder may realize the item is worth less than the price paid.

Here is a simple example:

A baseball card is being auctioned.
Everyone thinks it may be rare, but no one knows for sure.
Bidder A estimates the card is worth $100$.
Bidder B estimates $80$.
Bidder C estimates $60$.

If bidder A wins, that is a warning sign: A may have been the most optimistic, not the most accurate. If the card is actually worth only $70$, then A paid too much. In a common value auction, bidders often lower their bids to protect themselves from this risk.

This is why information matters. Better information can help a bidder make a smarter estimate and avoid paying more than the item is really worth. In some auctions, bidders try to gather signals before bidding, such as:

inspecting a property
reading market reports
using experts or engineers
studying past sales data

Private vs. Common Values in Simple Examples

Sometimes real goods are not perfectly private or perfectly common. But the two models help us understand the main logic.

Let’s compare some examples, students:

Example 1: A movie ticket

If you want to see a movie and your friend does not, the ticket is mostly a private value good. The value comes from your personal enjoyment.

Example 2: A rare comic book for resale

If everyone is trying to buy the comic book to sell it later, and the future resale price is uncertain, that is closer to a common value setting. The comic has one market value, but no one knows it perfectly.

Example 3: A used phone for your own use

If you want the phone because it has a camera you like and your current phone is broken, that is mainly private value. Your personal value depends on your needs.

Example 4: An oil drilling lease

This is a classic common value example because the amount of oil is hidden. Bidders may see different clues, but the actual value is tied to the same underground resource.

A useful test is this question:

Does each bidder care mostly about personal use, or about the same unknown future payoff?

If it is personal use, think private value. If it is about the same uncertain resale or payoff, think common value.

Mixed Situations: When Both Ideas Matter

Many real auctions are not purely one type. Instead, they contain both private and common elements.

For example, consider a house auction. The house may have a common component because the true market price is influenced by things everyone cares about, such as neighborhood trends and repair costs. But each bidder also has private value from personal preferences, such as liking the school district or wanting a bigger backyard.

This means two bidders can look at the same house and still value it differently because:

they have different private tastes, and
they may have different information about its true market value.

That is why economists often say value structure matters. It affects how people bid, how much they are willing to risk, and whether the final price ends up close to or far from the item’s true worth.

A simple way to organize the difference is:

Private value: the item is worth different amounts to different people because preferences differ.
Common value: the item has one true worth, but it is unknown.
Mixed value: both personal preference and uncertainty about true worth matter.

Conclusion

students, private and common values are two basic ways to describe uncertainty in auctions and markets. In a private value setting, each bidder has their own value, and that value depends on personal preferences. In a common value setting, everyone is trying to estimate the same unknown true value, so information becomes crucial. Better information can improve bidding decisions and reduce mistakes, especially in common value auctions where the winner’s curse can happen.

When you look at a real-world auction, ask two questions:

Is the value mostly personal?
Or is there one unknown value that everyone is trying to estimate?

Answering those questions helps you identify the structure of the situation and predict how people will bid. 🧠

Study Notes

Private values mean each bidder has a personal value for the item.
In private value auctions, one bidder’s value does not change because of another bidder’s information.
Common values mean the item has one underlying true value, but it is unknown to bidders.
In common value auctions, different bidders may have different signals about the same true value.
Information matters more in common value settings because bidders are estimating an unknown fact about the world.
The winner’s curse happens when the winning bidder is often the most optimistic and may overpay.
Examples of private values include concert tickets, clothing, and goods for personal use.
Examples of common values include oil leases, mineral rights, and many resale items.
Some situations are mixed, with both personal taste and uncertain market value.
To identify the value structure, ask whether bidders care about personal use or the same hidden future payoff.