theory that stock prices behave in an unpredictable fashion because the stock market is efficient. The market price of a stock goes randomly around real (intrinsic) value. Current security prices are independent of prior prices. Thus historical prices are not a reliable predictor of future ones.According to random walk, financial information significant enough to affect future value is available to knowledgable investors. Thus new data affecting stock prices are immediately reflected in market value. At any given time, the price of a stock is the optimum estimate of its value including all available information.