More than One-third of US stock market trading comes from unknown sources

Study Lays Bare Breadth of 'Naked' Access

The report by Aite Group, a Boston research outfit that tracks high-frequency trading, found that naked access — trading directly on exchanges using a brokerage's computer identification code — accounts for an estimated 38% of the U.S. stock market's average daily trading volume.

Naked access is one form of a more widespread practice called sponsored access in which brokers let trading firms operate on exchanges using the brokers' market participant identification codes. Overall, sponsored access accounts for half of the market's volume, according to the Aite report, which is likely to be released Monday.