2.9.1—Public goods

A pure public good is non-rival and non-excludable.

These properties create free riding; many real goods are mixed.

Test rivalry and exclusion separately before classifying.

National defence protects residents together; a crowded park is rival at capacity.

Publicly provided does not automatically mean public good.

Non-rivalry means one person's use does not reduce the amount available to others; non-excludability means non-payers cannot feasibly be prevented from benefiting. Because each person can wait for others to pay, stated willingness to pay is understated and a profit-seeking market cannot reliably collect enough revenue, causing non-provision or under-provision relative to the efficient quantity.