Abstract
There are three main possible sources of economic gain arising from the adoption of the Internal Market Programme: increased specialization in accordance with the law of comparative advantage, changes in economic efficiency brought about by increased competition, and increased production levels due to a better exploitation of economies of scale made possible by the increase in the size of the market. The aim of the work by Muller and Owen (1985) is to analyze the last source. More precisely they consider the effect of trade on the deviation of the representative plant size from the minimum efficient technical scale (METS) at the industrial level.