Short Sales and Trade Classification Algorithms

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This paper demonstrates that the Lee-Ready and other commonly used trade classification algorithms classify short sales as buyer-initiated significantly more than 50% of the time. This result is due in part to regulations which require short sales be executed on an uptick or zero-uptick. In addition, while the literature considers immediacy premiums in determining trade direction, they ignore the often larger borrowing premiums which short sellers must pay. Since short sales constitute approximately 30% of all trade volume on U.S. exchanges, these results are important to the empirical market microstructure literature as well as to measures that rely upon trade classification, such as the probability of informed trading (PIN) metric.