Random Walk Theory

What is Random Walk Theory and what are its implications for financial markets?

"Random Walk" is an economic theory that market price movements move randomly. This assumes an efficient market. The theory also assumes that new information comes to the market randomly. Together, the two assumptions imply that market prices move randomly as new information is incorporated into market prices. The theory implies that the best predictor of future prices is the current price, and that past prices are not a reliable indicator of future prices. If the random walk theory is correct, Technical Analysis cannot work. (Source: CFTC)

The random walk hypothesis is a financial theory stating that stock market prices evolve according to a random walk and thus the prices of the stock market cannot be predicted. It has been described as 'jibing' with the efficient market hypothesis. Investors, economists, and other financial behaviorists have historically given into the random walk hypothesis. They have run several tests and still believe that stock prices are completely random because of the efficiency of the market.

The term was popularized by the 1973 book, "A Random Walk Down Wall Street", by Burton Malkiel, currently a Professor of Economics and Finance at Princeton University.

Burton G. Malkiel, an economist professor at Princeton University and writer of A Random Walk Down Wall Street, did a test where his students were given a hypothetical stock that was initially worth fifty dollars. The closing stock price for each day was determined by a coin flip. If the result was heads the price would close a half point higher, and subsequently if the result was tails, it would close a half point lower. Each time there was a fifty-fifty chance of the price closing higher or lower than the previous day. There were cycles or trends determined from the tests. Malkiel then took the results in a chart and graph form to a chartist (a person who “seeks to predict future movements by seeking to interpret past patterns on the assumption that ‘history tends to repeat itself’”) (Keane 11). The chartist told Malkiel that they needed to immediately buy the stock. When Malkiel told him it was based purely on flipping a coin, the chartist was very unhappy. This indicates that the market and stocks could be just as random as flipping a coin.

The random walk hypothesis was also applied to NBA basketball. Psychologists did a detailed study of every shot the Philadelphia 76ers made over one and one-half seasons of basketball. The psychologists found no positive correlation between the previous shots and the outcomes of the shots afterwards. Economists and believers in the random walk hypothesis apply this to the stock market. The actual lack of correlation of past and present can be easily seen. If a stock goes up one day, no stock market participant can accurately predict that it will rise again the next. Just as a basketball player with the “hot hand” can miss his or her next shot, the stock that seems to be on the rise can fall at any time, making it completely random.

There are other economists, professors, and investors that believe that the market is predictable to some degree. The people believe that there are trends and incremental changes in the prices and when looking at them, one can determine whether the stock is on the rise or fall. There have been key studies done by economists and even a book written by two professors of economics that try to prove the random walk hypothesis wrong.

Martin Weber, a leading researcher in behavioral finance, has done many tests and studies on finding trends in the stock market. In one of his key studies, he observed the stock market for ten years. Over those ten years, he looked at the market prices and looked for any kind of trends. He found that stocks with high price increases in the first five years tended to become under-performers in the following five years. Weber and other believers in the non-random walk hypothesis cite this as a key contributor and contradictor to the random walk hypothesis.

Another test that Weber ran that contradicts the random walk hypothesis was finding stocks that have had an upward revision for earnings outperform other stocks in the forthcoming six months. With this knowledge, investors can have an edge in predicting what stocks to pull out of the market and what stocks, the stocks with the upward revision, to leave in. Martin Weber’s studies detract from the random walk hypothesis, because according to Weber there are trends and other tips to predicting the stock market. Professors Andrew W. Lo and Al. Craig MacKinlay professors of Finance at Sloan School of Management and the University of Pennsylvania, respectively, have also tried to prove theory wrong. They wrote the book A Non-Random Walk Down Wall Street, that goes through a number of tests and studies that try to prove that there are trends in the stock market and they are somewhat predictable. They argue that the random walk does not exist and that even the casual observer can look at the many stock and index charts generated over the years and see the trends. If the market were random, it is argued, there would never be the many long rises and declines so clearly evident in charts. Subscribers to the random walk hypothesis counter argue that past performance cannot be indicative of future performance in a semi-strong market economy. (Source: Wikipedia)