THIS PAPER is part of our continuing analysis of common stock price fluctuations, which we call volatility patterns.' Here, our focus is upon industrial stock price indices. We investigate (1) whether or not an industry can be classified according to its stock price volatility-i.e. whether a series of volatility statistics, each computed for a relatively short time span, will be fairly constant for individual industries over time, and (2) if inter-industry volatility differentials remain constant over time.Part I presents a brief discussion of general issues related to the topic of volatility. Part II summarizes the computational models, and Part III presents the empirical results yielded by the industry study. Part IV summarizes our findings.I. GENERAL ISSUES ON VOLATILITY In considering volatility, our goal is not to predict the direction of future price changes on the basis of historical data,2 but rather to assess whether price fluctuations (regardless of direction) are likely to occur. We hypothesize that, over a succession of time periods, the volatility of prices will be reasonably constant. In our earlier firm specific analysis of price fluctuations [1], we did observe that, for some firms, stock prices were quite volatile relative to the fluctuations exhibited by other firms. In this present paper, we extend the analysis to industry groupings of firms. Evidence that some industries are consistently more volatile than others would suggest that traditional groupings of firms by industry are useful for analytical purposes.3 In addition, knowledge of volatility patterns would be useful information for the formulation of investment strategies and for portfolio management.This study of short run volatility patterns should be of particular interest to * Associate Professors of Economics and Finance, Graduate School of Business Administration, New York University. Edward I. Altman was a Visiting Professor of Finance at the Center d'Enseignement Superieure des Affaires in France during the 1971-72 academic year. The authors wish to acknowledge the research assistance of Allistair Hunter Henderson, and the helpful comments of an unknown referee.1. In a prior paper [1], we present two basic models employed to measure volatility, and several .approaches to testing for stability. In this earlier work, we were concerned with the empirical measurement of the volatility behavior of specific firms' stock price movements. We utilize the same measures of volatility in the current paper, and will review them in Part II.
See Cootner[51 for a collection of major historical works in the random walk literature, and Fama [7] for a discussion of the literature on efficient capital market models and related empirical tests. 3. Sharpe [17] and King [11], among others, have shown that economic and financial information relates, primarily, either to specific firms, to industries, or to the aggregate market. Also see Elton and Gruber [6] for a further analysis of the meaningfulness of industry groupings. 957 958 The Journal of Finance