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An empirical test of the theory of random walks in stock market prices : the moving average strategy Yip, Garry Craig


This study investigates the independence assumption of the theory of random walks in stock market prices through the simulation of the moving average strategy. In the process of doing so, three related questions are examined: (1) Does the past relative volatility of a stock furnish a useful indication of its future behavior? (2) Is the performance of the decision rule improved by applying it to those securities which are likely to be highly volatile? (3) Does positive dependence in successive monthly price changes exist? The purpose of Test No. 1 was to gauge the tendency for a stock's relative volatility to remain constant over two adjacent intervals of time. As measured by the coefficient of variation, the volatility of each of the 200 securities was computed over the 1936 to 1945 and 1946 to 1955 decades. In order to evaluate the strength of the relationship between these paired observations, a rank correlation analysis was performed. The results indicated a substantial difference in relative volatility for each security over the two ten-year periods. In Test No. 2 a different experimental design was employed to determine whether the relative volatility of a stock tended to remain within a definite range over time. According to their volatility in the 1936 to 1945 period, the 200 securities were divided into ten groups. Portfolio No. 1 contained the twenty most volatile securities while Portfolio No. 2 consisted of the next twenty most volatile, etc. An average coefficient of variation was calculated for each group over the periods, 1936 to 1945 and 1946 to 1955. The rank correlation analysis on these ten paired observations revealed that the most volatile securities, as a group, tended to remain the most volatile. Test No. 3 consisted of the application of the moving average strategy (for long positions only) to forty series of month-end prices covering the interval, 1956 to 1966. These securities had demonstrated a high relative volatility over the previous decade and, on the basis of the findings reported in Test No. 2, it was forecasted that they would be the most volatile of the sample of 200 in the period under investigation. Four different moving averages ranging from three to six months, and thirteen different thresholds ranging from 2 to 50 per cent were simulated. The results of the simulation showed the moving average strategy to be much inferior to the two buy-and-hold models. Every threshold regardless of the length of the moving average yielded a negative return. In addition, the losses per threshold were spread throughout the majority of stocks. Altogether, therefore, considerable evidence was found in favour of the random walk theory of stock price behavior.

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