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Cooperation among human beings have existed in different forms from past centuries till now. The modern cooperatices have been formed in response to disorders due to the occurrence of industrial revolution. Inefficiency of market and government for various reasons, justifies the formation of the third sector of economy (including cooperatives). This paper aims to answer the following questions:What are the reasons for the formation of cooperatives? What is the thire sector of economy? What are the roles of cooperatives in macroe conomy?

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The purpose of this study is to introduce an index for calculating transaction costs in Iran over the 1984-2006 period. In order to calculate the transaction costs, Fuzzy Logic Approach is employed using MATLAB software. Based on the literature review, the determinants of the transaction costs are identified. They mainly comprise the standard deviation of the inflation rate, economic freedom, cash ratio and the ratio of public costs to GDP. Three scenarios are considered to calculate the transaction costs. Overall, the result shows that there is a cyclical trend in the transaction costs over the period considered in this study. The transaction costs decreased after the period of war - 1372-1374 - in all three scenarios considered. Following this period, however, the transaction costs has an upward trend and continued to increase until 1381 in all scenarios. After this increasing trend, the transaction costs have again decreased in the three scenarios.