Our procedure splits the volume in a time bar equally between buy and sell volume if there is no price change from the beginning to the end of the time bar. Alternatively, if the price increases, the volume is weighted more toward buys than sells, and the weighting depends on how large the price change is relative to the distribution of price changes.
A key difference between bulk classification and the Lee-Ready algorithm is that the latter signs volume as either buy or sell, while the former signs a fraction of the volume as buys and the remainder as sells.13 In other words, the Lee-Ready algorithm provides a discrete classification, while the bulk algorithm is continuous. This means that even in the extreme case that a single time bar fills a volume bucket, volume may still be perfectly balanced according to bulk classification