It has been more than 50 years since the idea of stock prices containing all relevant information was put forth. Information might come in the form of data from a company’s financial statements, news about a new product, a change in the regulatory environment, or simply a shift of investors’ tastes and preferences toward owning different investments.
Information is incorporated into security prices through the buying and selling process. While fair prices may not depend on a certain level of trading, over $400 billion of stocks traded on average each day in the world equity markets suggests that a great deal of information is incorporated into stock prices.