Stock Price Movements Are Unpredictable


Burton Malkiel (BM): Basically, the concept is not that the market is random or capricious, really, just the opposite: that the market is quite efficient in reflecting new information and when news arises that’s true news, the market adjusts without delay. The true news is unpredictable—in other words, if we have a headline today that says “New York digs out of yesterday’s storm,” that’s not news. What was news is that the storm was much bigger than anybody had predicted. So the true news is random or unpredictable. It’s something that you didn’t know before, such as “Egypt is in crisis.” The markets will then react without delay. But since you can’t predict true news, the market is generally unpredictable. It’s not that it’s capricious; quite the contrary: It’s that it reacts to unexpected events. And if you could predict the unexpected events, you could predict the market. But since you can’t, markets are unpredictable.

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