national defense; free-rider) • Non-rivalry, non-excludability, congestion: environmental public good with consumption externalities (excessive consumption because consumers ignore external costs; for example, pollution of a lake or the air) • Rivalry, non-excludability, non- congestion: free good (supply exceeds demand at zero price) )• Rivalry, non-excludability, congestion: open access and commonly owned resources (users own the good in common; consumers respond to marginal private costs rather than at marginal social costs; inefficient overconsumption, underinvestment in preserving the stock of goods that will be consumed by another, for example the forest used for firewood) • In summary, the efficiency implications of the various factors; The types of market failures involving public goods are as follows: o In tolling (non-rivalry/excludability) situations, underconsumption results from economically inefficient pricing rather than a lack of supply; this problem is exacerbated by congestion which results in the need for variable prices to reach