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The extent to which the price of an asset reflects all information available. Economists disagree on how efficient markets are. Followers of the efficient markets theory hold that the market efficiently deals with all information on a given security and reflects it in the price immediately, and that technical analysis, fundamental analysis, and/or any speculative investing based on those methods are useless. On the other hand, the primary observation of behavioral economics holds that investors (and people in general) make decisions on imprecise impressions and beliefs, rather than rational analysis, rendering markets somewhat inefficient to the extent that they are affected by people.
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economic efficiencyan aspect of PRODUCTION that seeks to identify, for a given level of OUTPUT, the combination of FACTOR INPUTS that minimizes the COST of producing that output. More broadly, economic efficiency is equated with the effectiveness of RESOURCE ALLOCATION in the economy as a whole such that outputs of goods and services fully reflect consumer preferences for these goods and services as well as individual goods and services being produced at minimum cost through appropriate mixes of factor inputs.
Collins Dictionary of Economics, 4th ed. © C. Pass, B. Lowes, L. Davies 2005