Power law for the calm-time interval of price changes

In this paper, we describe a newly discovered statistical property of time series data for daily price changes. We investigated quantitatively the calm-time intervals of price changes for 800 companies listed in the Tokyo Stock Exchange, and for the Nikkei 225 index in the 27-year period from January 1975 to December 2001. A calm-time interval is defined as the interval between two successive price changes above a fixed threshold. We found that the calm-time interval distribution of price changes obeys a power-law decay. Furthermore, we show that the power-law exponent monotonically decreases with respect to the threshold.