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When it is impossible to predict the next step. As per the Efficient Market Theory the prices of financial assets (such as shares) follow a random walk– there is no way of knowing the next change in the price. The reason this theory provides 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. Such belief has led some economists to conclude that investors cannot outperform the market consistently.
As opposed to this, some economists argue that asset prices are predictable and that markets are not efficient–they follow a non-random walk perspective.