Economic theory has produced many dynamic models of economic growth and development, some of which take into account the export-import sector and balance of payments problem so vital to underdeveloped countries. These models are either product-market oriented, money-market oriented, or some combination of both. Those that are product-market oriented generally take the import sector as exogenously determined or changing at some specific, usually constant, rate and then analyze the export sector with respect to it; suggesting that in accordance with the theory of comparative advantage, an underdeveloped country should expand its production of export good or goods in which it possesses a comparative advantage, as a means of improving its balance of payments position and moving towards sustained economic growth. Some of these dynamic theoretical models have not been tested with actual data from underdeveloped nations, largely due to the problem of obtaining reliable data from these countries. Those that have been tested, have used, for the most part, data that ranged over only a short period of time, due to the problem stated above. Several of the more important contributions are summarized in the following section.