There is another way to choose 20 stocks for your portfolio: Pick them randomly by, for instance, putting the stock pages on your bulletin board and throwing darts at the page. This may sound crazy, but there is reason to believe that it won’t lead you too far astray. That reason is called the efficient markets hypothesis. To understand this theory, the starting point is to acknowledge that each company listed on a major stock exchange is followed closely by many money managers, such as the individuals who run mutual funds. Every day, these managers monitor news stories and conduct fundamental analysis to try to determine the stock’s value. Their job is to buy a stock when its price falls below its value and to sell it when its price rises above its value The second piece to the efficient markets hypothesis is that the equilibrium of supply and demand sets the market price. This means that. at the market price, the number of shares being offered for sale exactly equals the number of shares that people want to buy. In other words, at the market price, the number of people who think the stock is overvalued exactly balances the number of people who think it’s undervalued. As judged by the typical person in the market, all stocks are fairly valued all the time. According to this theory, the stock market is information-ally efficient: It reflects all available information about the value of the asset. Stock prices change when information changes. When good news about the company’s prospects becomes public, the value and the stock price both rise. When the company’s prospects deteriorate, the value and price both fall. But at any moment in time, the market price is the best guess of the company’s value based on available information. One implication of the efficient markets hypothesis is that stock prices should follow a random walk. This means that the changes in stock prices are impossible to predict from available information. If, based on publicly available information, a person could predict that a stock price would rise by 10 percent tomorrow, then the stock market must be failing to incorporate that information today. According to this theory, the only thing that can move stock prices is news that changes the market’s perception of the company’s value. But news must be unpredictable otherwise, it wouldn’t really be news. For the same reason, changes in stock prices should be unpredictable. If the efficient markets hypothesis is correct, then there is little point in spending many hours studying the business page to decide which 20 stocks to add to your portfolio. If prices reflect all available information, no stock is a better buy than any other. The best you can do is buy a diversified portfolio.