Random Walk Theory and Example

What Is the Random Walk Theory?

Random walk theory suggests that changes in stock prices have the same distribution and are independent of each other. Therefore, it assumes the past movement or trend of a stock price or market cannot be used to predict its future movement. In short, random walk theory proclaims that stocks take a random and unpredictable path that makes all methods of predicting stock prices futile in the long run.

Understanding Random Walk Theory

Random walk theory believes it's impossible to outperform the market without assuming additional risk. It considers technical analysis undependable because chartists only buy or sell a security after an established trend has developed. Likewise, the theory finds fundamental analysis undependable due to the often-poor quality of information collected and its ability to be misinterpreted. Critics of the theory contend that stocks do maintain price trends over time – in other words, that it is possible to outperform the market by carefully selecting entry and exit points for equity investments.

Efficient Markets are Random

The random walk theory raised many eyebrows in 1973 when author Burton Malkiel coined the term in his book "A Random Walk Down Wall Street." The book popularized the efficient market hypothesis (EMH), an earlier theory posed by University of Chicago professor William Sharp. The efficient market hypothesis states that stock prices fully reflect all available information and expectations, so current prices are the best approximation of a company’s intrinsic value. This would preclude anyone from exploiting mispriced stocks consistently because price movements are mostly random and driven by unforeseen events.

Sharp and Malkiel concluded that, due to the short-term randomness of returns, investors would be better off investing in a passively managed, well-diversified fund. A controversial aspect of Malkiel’s book theorized that "a blindfolded monkey throwing darts at a newspaper's financial pages could select a portfolio that would do just as well as one carefully selected by experts."

Random Walk Theory in Action

The most well-known practical example of random walk theory occurred in 1988 when the Wall Street Journal sought to test Malkiel's theory by creating the annual Wall Street Journal Dartboard Contest, pitting professional investors against darts for stock-picking supremacy. Wall Street Journal staff members played the role of the dart-throwing monkeys.

After more than 140 contests, the Wall Street Journal presented the results, which showed the experts won 87 of the contests and the dart throwers won 55. However, the experts were only able to beat the Dow Jones Industrial Average (DJIA) in 76 contests. Malkiel commented that the experts' picks benefited from the publicity jump in the price of a stock that tends to occur when stock experts make a recommendation. Passive management proponents contend that, because the experts could only beat the market half the time, investors would be better off investing in a passive fund that charges far lower management fees.