# Public, Private, and Common Goods

> AP Microeconomics · AP Microeconomics
> Source: https://www.owlsprep.com/study/ap-microeconomics-u6-public-private-and-common-goods/

This module breaks down 4 core economic good categories using excludability and rivalrousness criteria, plus their associated market failures including free riding and the tragedy of the commons.

**Prerequisites:** [Introduction to market failure definitions](https://www.owlsprep.com/study/ap-microeconomics-u6-introduction-to-market-failure/); [Marginal social benefit and marginal social cost](https://www.owlsprep.com/study/ap-microeconomics-u6-positive-and-negative-externalities/)

## Learning objectives

- Distinguish between excludability and rivalrousness as the two core criteria for classifying economic goods
- Categorize real-world goods into private, public, common, and artificially scarce (club) groups
- Explain the free-rider problem and its role in causing under-provision of public goods by free markets
- Analyze the tragedy of the commons as the key market failure for non-excludable rival common resources

## Core Classification Criteria for Economic Goods

All economic goods are sorted using two independent, binary properties, no exceptions. You will never see a good that is partially excludable or partially rival on the AP exam, all testable examples fall cleanly into one of four quadrants.

**Excludability** — A good is excludable if the producer can easily stop someone who does not pay for the good from using it. For example, a movie theater can turn away non-ticket holders, so theater seats are excludable.

**Rivalrousness** — A good is rival if one person consuming a unit of the good means no other person can consume that exact same unit. For example, if you eat a sandwich, no one else can eat that same sandwich, so it is rival.

**Worked example:** Classify each of the following two properties for a streetlight installed on a public sidewalk: (1) Is it excludable? (2) Is it rivalrous?

1. Step 1: Test excludability. Can you stop a person walking past the streetlight from benefiting from its light if they did not pay for it? There is no practical way to do this, so streetlight illumination is non-excludable.
2. Step 2: Test rivalrousness. If one person walks under the streetlight and uses its light to see, does that reduce the amount of light available for other people walking past? No, so streetlight illumination is non-rival.

**Check your understanding**

1. Which of the following is true of a good that is non-excludable and rival?

   - It is a private good
   - It is a common resource
   - It is a pure public good
   - It is a club good

   *Why:* Non-excludable and rival is the exact definition of a common resource.

## Private Goods and Pure Public Goods

Private goods are both excludable and rival. These are the standard goods you buy at a grocery store or retail shop, and competitive free markets typically allocate them efficiently with no inherent market failure. Pure public goods are the exact opposite: they are both non-excludable and non-rival.

**Free Rider Problem** — A market failure that occurs for non-excludable public goods, where individual consumers choose not to pay for the good because they can access it for free once someone else pays for its production.

**Worked example:** A small town with 100 residents is considering building a public lighthouse that costs \$10,000 total. Each resident values the lighthouse at \$150. Explain why the lighthouse will not be provided by a private for-profit firm even though total social benefit exceeds total social cost.

1. Step 1: Calculate total social benefit. 100 residents × \$150 valuation = \$15,000, which is \$5,000 higher than the \$10,000 total cost.
2. Step 2: Identify the free rider incentive. If a private firm tries to sell lighthouse access for \$100 per person, every resident will choose not to pay, because they know if enough other people pay, they can use the lighthouse for free. No one will voluntarily contribute, so the private firm cannot raise the \$10,000 required to build it.
3. Step 3: Conclusion. The unregulated free market will produce zero lighthouses, which is the inefficient under-provision outcome caused by the free rider problem.

**Exam command terms**

AP Micro exam questions use specific command terms for this topic:

- **Classify** — You must explicitly state both excludability and rivalrousness for the good to get full points *("Classify national defense as a public good" requires you to note it is non-excludable and non-rival.)*

- **Explain** — You must link the good's classification properties directly to the associated market failure, no vague statements

## Common Resources and the Tragedy of the Commons

Common resources are non-excludable but rival. No one can be stopped from using them, but every unit one person uses reduces the units available for everyone else. This unique combination leads directly to the tragedy of the commons market failure, where the resource is overused and eventually depleted far past the socially optimal level.

**Worked example:** An unregulated ocean fishing ground has no limits on how many fish each commercial boat can catch. Explain why the total catch will be higher than the socially efficient quantity.

1. Step 1: Identify the missing incentive. Individual fishing boats only consider their own private marginal cost of catching fish, and do not account for the fact that every fish they catch reduces the fish available for all other boats, raising the marginal cost for every other fisher.
2. Step 2: Compare private and social cost. The marginal social cost of fishing is higher than the private marginal cost faced by individual boats. The market equilibrium quantity occurs where private marginal cost equals marginal benefit, which is a higher quantity than the socially optimal quantity where social marginal cost equals marginal benefit.
3. Step 3: Outcome. Over time, the total fish population collapses, a classic tragedy of the commons outcome.

> **warning**
>
> Never mix up the free rider problem and the tragedy of the commons on the exam. Free rider applies to non-rival public goods, tragedy of the commons applies to rival common resources. Using the wrong term will cost you all points for that part of the FRQ.

## Artificially Scarce (Club) Goods

The fourth and final category of goods is artificially scarce or club goods, which are excludable but non-rival. These goods have zero marginal cost to provide access to one additional consumer, so charging a positive price for them creates deadweight loss from under-consumption.

**Comparing methods**

Use this quick reference to sort any good on the exam:

- **Private Good** — Excludable + Rival, no inherent market failure

- **Public Good** — Non-excludable + Non-rival, free rider problem

- **Common Resource** — Non-excludable + Rival, tragedy of the commons

- **Club Good** — Excludable + Non-rival, under-consumption deadweight loss

**Worked example:** Classify the following three goods: (1) Netflix subscription, (2) Loaf of bread, (3) Public fireworks display

1. Step 1: Netflix is excludable (you need a paid account to access it) and non-rival (one person watching does not reduce content availability for others), so it is a club good.
2. Step 2: A loaf of bread is excludable (you have to pay to take it from the store) and rival (if you eat it no one else can), so it is a private good.
3. Step 3: A public fireworks display is non-excludable (you can watch from your backyard without paying) and non-rival (one person watching does not block the view for others), so it is a pure public good.

## Common pitfalls

- **Wrong:** Calling any non-rival good a public good even if it is excludable
  - Why it fails: Pure public goods require BOTH non-excludable and non-rival properties, not just one. Excludable non-rival goods are club goods, not public goods.
  - Correct: Verify both criteria explicitly before classifying any good to avoid half points on FRQs.
- **Wrong:** Stating all goods provided by the government are pure public goods
  - Why it fails: Governments regularly provide rival, excludable private goods like public school meals or public housing that are not pure public goods.
  - Correct: Classify goods by their inherent consumption properties, not by who produces or funds them.
- **Wrong:** Confusing the free rider problem with the tragedy of the commons
  - Why it fails: The two market failures apply to completely different good categories, and AP graders will reject your answer if you use the wrong term.
  - Correct: Map the problem to the good's classification first: free rider for non-rival public goods, tragedy of commons for rival common resources.
- **Wrong:** Assuming all public roads are pure public goods
  - Why it fails: Congested roads have positive rivalry, as one extra car reduces space for other drivers, so they become common resources, not pure public goods.
  - Correct: Check for congestion or consumption rivalry before classifying infrastructure as a public good.
- **Wrong:** Claiming private goods can never generate positive externalities
  - Why it fails: Private goods like vaccines are fully rival and excludable, but still create large positive spillover benefits for third parties.
  - Correct: Keep good classification and externality analysis as separate, independent frameworks.

## Cheatsheet

| Good Type | Excludable? | Rivalrous? | Key Market Failure | Real World Example |
| --- | --- | --- | --- | --- |
| Private Good | Yes | Yes | No inherent failure | Grocery store apple |
| Public Good | No | No | Free rider, under-provision | Coastal lighthouse |
| Common Resource | No | Yes | Tragedy of the commons, overuse | Unregulated ocean fish |
| Club (Artificially Scarce) Good | Yes | No | Under-consumption DWL | Paid streaming service |

## What's next

Mastering good classification is a critical foundational skill for the rest of Unit 6, as it directly informs your understanding of government intervention policies to correct market failures. This topic is a very frequent source of standalone FRQ part (a) questions on the AP Micro exam, so ensure you can classify any given good in 10 seconds or less to save time for longer calculation questions later. You will next learn how to design targeted policies to address the free rider problem for public goods, including cost-benefit analysis for government provision, and regulation systems like cap-and-trade or individual transferable quotas to mitigate the tragedy of the commons for common resources.

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