Before revision:  Up to 23\% of observed trades may have contributed the the measured inefficiencies , leading to a ROC greater than 2 billion USD. A subset of the constituents of the S&P 500 index experience the greatest amount of ROC and may drive inefficiencies in other stocks.
After revision:  Up to 23\% of observed trades may have contributed the the measured inefficiencies , leading to estimated opportunity costs on the order of 2 billion USD. A subset of the constituents of the S&P 500 index experience the greatest amount of ROC and may drive inefficiencies in other stocks.