Quote from ammo:
if you look up archipelago ,the first electronic clearing firm, and its eventual selling to a handful of large houses,the research will tell you that your acct has a number like your ss #,it is in the system,it also says whether your long or short and avg size you trade,where your stops are and so on,now if the larger houses have all this data ,and they do a little research they can come up with an avg retail long or short and stops and run it to the most profitable spot,they can also differentiate between the 1 lot n 10 lot scalper and the massive positioned hedge funds in the same manner,it's not public knowledge but it's not insider info unless they sell it (i think)so they basically own the proverbally golden goose,