根据下列短文，回答题。
　　There are various in which individual economic units Can interact with one another．Three basic ways may be described as the market system，the administered system and the traditional system．In a market system individual economic units are free to interact among each other in themarketplace．It is possible to buy commodities from other economic units or sell commodities tothem．In a market，transactions may take place via barter or money exchange．In a barter economy，real goods such as automobiles，shoes and pizzas are traded against each other．Obviously，finding somebody who wants to bade my old Car in exchange for a sailboat may not always bean easy task．Hence，the introduction of money as a medium of exchange eases transactions considerably．In the modern market economy，goods and services are bought or sold for money．
　　An alternative to the market system is administrative control by some agency over all trans．actions．This agency will issue edicts or commands as to how much of each good and service should be produced，exchanged，and consumed by each economic unit．Central 01anaing may be one way of administering such an economy．The central plan，drawn up by the government,shows the amounts of each commodity produced by the various firms and allocated to differenthouseholds for consumption．This is an example of complete planing of production，consump—tion，and exchange for the whole economy．
　　In a traditional society，production and consumption paUerns are governed by tradition：everyperson’S place within the economic system is fixed by parentage，religion and custom．　　　　　Transac—tions take place on the basis of tradition，too．People belonging to a certain group or caste mayhave an obligation to care for other persons，provide them with food and shelter，care for theirhealth，and provide for their education．Clearly，in a system where every decision is made on thebasis of tradition alone，progress may be difficult to achieve．A stagnant society may result．
What is main purpose of the passage?