Contrast the reference prices used to judge trade execution

Pre-trade benchmarks are known before trading starts: the arrival price, the mid-quote when the order reaches the trader, suits urgent orders and separates delay cost from execution cost within implementation shortfall, which is itself measured from the decision price, while the previous close is sometimes used for index-related trades. Intraday benchmarks, VWAP and TWAP, suit orders worked through the day; VWAP can be gamed through timing and means little when the order is a large share of volume. The closing price is the natural benchmark for index funds and funds valued at the close. Post-trade benchmarks, such as a later closing price, reveal price reversion after the trade. The trap is using VWAP for a very large order: the order moves VWAP itself, making execution look better than it really was.