老师您好，请问一个题：
Craig is analyzing a trade in which Placid had entered an order to buy shares of ABC stock at market. Dealer price and volume information at the time of the trade order and trade execution are provided in Exhibit 1.
order bid price ask price
entry 20 20.04
excution 19.97 20.03
问题:Based on Exhibit 1, the effective spread for the executed ABC stock trade is most likely:
1.smaller
2. equal to the quoted bid–ask spread.
3. greater than the quoted bid–ask spread.
答案：1
解释：
Effective spread is a measure of execution costs that captures both the effects of price improvements and market impact. The effective spread is two times the deviation of the actual execution price from the mid-point of the market quote at the time an order is entered.
Effective spread = 2 × (Actual execution price − Mid-point of market quote at time of order entry)
= 2 × ($20.03 − $20.02) = 2 cents
Quoted spread = $20.04 − $20.00 = 4 cents
The price improvement from the trade has resulted in an effective spread that is lower than the quoted spread by 2 cents (0.02 effective spread − 0.04 quoted spread).
我的问题：actual execution 为什么是20.03？明明计算价差用的都是 entry那一栏的数据
