The Effective/Quoted Ratio compares the average effective spread with the quoted spread, or the difference between National Best Bid and Offer (NBBO) at time of order entry and a firm's trade execution. When an order is executed exactly at the NBBO, the Effective/Quoted Ratio is 100%.
Source: S3, a third-party analytics provider. All figures are based on publicly available 605 statistics involving orders of 100–1,999 shares. Figures include S&P 500 stocks only, for the 3rd quarter ending September 30, 2020.