Depends on the exact study and methodology, but generally the member firms and brokers each are aware of the trade access that they provide to specific clients, and in turn report to the exchanges.

This thread is a duplicate of the existing HFT thread:

HFT High Frequency Trading So i keep hearing traders complain about HFT, about how its changing everything. There's a ton of posts about it all over the web, Cramer even complains about it alot. I want to know how many people really believe that trading has become difficult …

Closing.

Mike