Economic Theory in General, and Trade Theory in Particular Are

Economic theory in general, and trade theory in particular are replete with equivalencies. For example, it is argued that for any specific tariff one can find an equivalent ad valorem tariff; and that for any quota one can calculate a tariff equivalent. Discuss conditions or situations under which a specific and an ad valorem tariff are not equivalent. Discuss conditions or situations when a tariff and a quota are not equivalent.