The method of controlling externality by laying down standards for polluting activities. Firms may, for example, be required to clean waste products up to some minimum standard before discharging them into rivers or sewers, or there may be standards of maximum tolerated emissions from road vehicles. Standards are contrasted with the use of the price mechanism to control pollution, by taxing emissions of polluting substances. A disadvantage of emission standards is that in order to avoid crippling economic activity, some level of pollution must be allowed: having reached this, a standard gives firms no incentive to reduce pollution further. Also, the costs of avoiding pollution are higher for some firms than others: a tax will reduce emissions where they are most cheaply avoided, whereas under a standard the marginal cost of reducing pollution to the permitted level may vary widely between firms.

• two-gap model
• two-part tariff
• two-stage least squares
• two-tier board
• type I and II errors
• unbiased estimator
• unbundling
• uncertainty
• uncompetitive
• UNCTAD
• undated security
• under-capitalized
• under-developed country
• under-funded pension scheme
• underlying rate of inflation
• under-subscription
• under-valued currency
• underwriting
• undistributed profits
• UNDP
• unearned income
• unemployable
• unemployment
• unemployment benefit
• unemployment rate