Impossible to predict the next step. Efficient market theory says that the prices of many financial assets, such as shares, follow a random walk. In other words, there is no way of knowing whether the next change in the price will be up or down, or by how much it will rise or fall. The reason is that in an efficient market, all the information that would allow an investor to predict the next price move is already reflected in the current price. This belief has led some economists to argue that investors cannot consistently outperform the market. But some economists argue that asset prices are predictable (they follow a non-random walk) and that markets are not efficient.