Public, Private, and Common Goods
AP MicroeconomicsΒ· 12 min read
1. Core Classification Criteria for Economic Goodsβ β ββββ± 15 min
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.
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.
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
Reveal answer
It is a common resource βNon-excludable and rival is the exact definition of a common resource.
2. Private Goods and Pure Public Goodsβ β β βββ± 18 min
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.
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.
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Step 1: Calculate total social benefit. 100 residents Γ $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.
3. Common Resources and the Tragedy of the Commonsβ β β βββ± 15 min
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.
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Step 3: Outcome. Over time, the total fish population collapses, a classic tragedy of the commons outcome.
4. Artificially Scarce (Club) Goodsβ β ββββ± 10 min
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
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.
5. Common Pitfalls
Wrong move:
Calling any non-rival good a public good even if it is excludable
Why:
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 move:
Verify both criteria explicitly before classifying any good to avoid half points on FRQs.
Wrong move:
Stating all goods provided by the government are pure public goods
Why:
Governments regularly provide rival, excludable private goods like public school meals or public housing that are not pure public goods.
Correct move:
Classify goods by their inherent consumption properties, not by who produces or funds them.
Wrong move:
Confusing the free rider problem with the tragedy of the commons
Why:
The two market failures apply to completely different good categories, and AP graders will reject your answer if you use the wrong term.
Correct move:
Map the problem to the good's classification first: free rider for non-rival public goods, tragedy of commons for rival common resources.
Wrong move:
Assuming all public roads are pure public goods
Why:
Congested roads have positive rivalry, as one extra car reduces space for other drivers, so they become common resources, not pure public goods.
Correct move:
Check for congestion or consumption rivalry before classifying infrastructure as a public good.
Wrong move:
Claiming private goods can never generate positive externalities
Why:
Private goods like vaccines are fully rival and excludable, but still create large positive spillover benefits for third parties.
Correct move:
Keep good classification and externality analysis as separate, independent frameworks.
6. Quick Reference 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 |
7. Frequently Asked
Why are pure public goods not efficiently provided by the unregulated free market?
Because non-excludability means private firms cannot charge non-paying consumers, so they cannot recoup production costs, leading to output far below the socially optimal quantity.
Is a public highway always classified as a pure public good?
No: if the highway is heavily congested, one additional car reduces available space for other drivers, making the good rivalrous. Congested highways are classified as common resources, not pure public goods.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2024 Β· Set 1 FRQ
Classify 3 goods and explain free rider issue
- 2022 Β· MCQ Section
Identify non-excludable rival good
- 2021 Β· Set 2 FRQ
Tragedy of the commons fishing scenario
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.