Exchanges are scrambling to cater to the high-frequency crowd, in part because the huge volumes they generate can make it easier for other traders on the exchange to execute orders.

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Direct Edge and Bats have successfully siphoned double-digit volume away from Nasdaq OMX and NYSE Euronext in recent years, in part by appealing to high-frequency trading firms with fast execution systems.

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Some investors use ETFs as a quick way of expressing their overall view on the market, while high-frequency traders use the funds as part of their complex arbitrage strategies.

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