Professor Gautam Kaul and two former PhD students, in their seminal 1994 study titled, "Transactions, Volume, and Volatility" convincingly argued and verified empirically that it is the occurrence of a trade in a certain direction rather than its dollar value (or volume) that has the greatest effect on prices, hence the greatest relevance when assessing the liquidity of the market where that trade took place. A trade sign is determined by the buyer or seller's information, while market conditions determine trade amount and price. This is a simple yet extremely powerful notion that was originally predicated in theory but had no empirical support before their 1994 study. The publication of this study opened the door to the accurate measurement and needed assessment of market liquidity. These days, the approach they recommended is widespread in its use.