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"Frontmatter". In: Analysis of Financial Time Series

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208 HIGH-FREQUENCY DATAthe time duration between price changes. <strong>In</strong> addition, let N i be the number <strong>of</strong> tradesin the time interval (t i−1 , t i ) that result in no price change. This new variable is usedto represent trading intensity during a period <strong>of</strong> no price change. Finally, let D i bethe direction <strong>of</strong> the ith price change with D i = 1 when price goes up and D i =−1when the price comes down, and let S i be the size <strong>of</strong> the ith price change measuredin ticks. Under the new definitions, the price <strong>of</strong> a stock evolves over time byP ti = P ti−1 + D i S i , (5.46)and the transactions data consist <strong>of</strong> {t i , N i , D i , S i } for the ith price change. ThePCD model is concerned with the joint analysis <strong>of</strong> (t i , N i , D i , S i ).Remark: Focusing on transactions associated with a price change can reducethe sample size dramatically. For example, consider the intraday data <strong>of</strong> IBM stockfrom November 1, 1990 to January 31, 1991. There were 60,265 intraday trades, butonly 19,022 <strong>of</strong> them resulted in a price change. <strong>In</strong> addition, there is no diurnal patternin time durations between price changes.To illustrate the relationship among the price movements <strong>of</strong> all transactions andthose <strong>of</strong> transactions associated with a price change, we consider the intraday tradings<strong>of</strong> IBM stock on November 21, 1990. There were 726 transactions on that dayduring the normal trading hours, but only 195 trades resulted in a price change. Figure5.14 shows the time plot <strong>of</strong> the price series for both cases. As expected, the priceseries are the same.The PCD model decomposes the joint distribution <strong>of</strong> (t i , N i , D i , S i ) given F i−1asf (t i , N i , D i , S i | F i−1 )= f (S i | D i , N i ,t i , F i−1 ) f (D i | N i ,t i , F i−1 ) f (N i | t i , F i−1 ) f (t i | F i−1 ).(5.47)This partition enables us to specify suitable econometric models for the conditionaldistributions and, hence, to simplify the modeling task. There are many ways tospecify models for the conditional distributions. A proper specification might dependon the asset under study. Here we employ the specifications used by McCulloch andTsay (2000), who use generalized linear models for the discrete-valued variables anda time series model for the continuous variable ln(t i ).For the time duration between price changes, we use the modelln(t i ) = β 0 + β 1 ln(t i−1 ) + β 2 S i−1 + σɛ i , (5.48)where σ is a positive number and {ɛ i } is a sequence <strong>of</strong> iid N(0, 1) random variables.This is a multiple linear regression model with lagged variables. Other explanatoryvariables can be added if necessary. The log transformation is used to ensure thepositiveness <strong>of</strong> time duration.

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