甚麼是Efficient markets? 怎樣區分?

In finance, the efficient market hypothesis (EMH) asserts that financial markets are "informationally efficient", or that prices on traded assets, e.g., stocks, bonds, or property, already reflect all known information and therefore are unbiased in the sense that they reflect the collective beliefs of all investors about future prospects. Professor Eugene Fama at the University of Chicago Graduate School of Business developed EMH as an academic concept of study through his published Ph.D. thesis in the early 1960s at the same school.
The efficient market hypothesis states that it is not possible to consistently outperform the market by using any information that the market already knows, except through luck. Information or news in the EMH is defined as anything that may affect stock prices that is unknowable in the present and thus appears randomly in the future.
http://en.wikipedia.org/wiki/Efficient_market_hypothesis