If you're a human, nine times out of ten the other side of your trade is an algorithm.

If you're an algorithm, you were programmed by humans. Those humans, by the way, thought the crash of 2008 was a "six Sigma" event, with odds of 1 in 500 million (or getting "heads" in 29 consecutive coin flips).

A subset of quantitative trading known as high-frequency trading accounted for 52 percent of May's average daily trading volume of about 6.73 billion shares, Tabb said. During the peak levels of high-frequency trading in 2009, about 61 percent of 9.8 billion of average daily shares traded were executed by high-frequency traders.

"His division embodies Vanguard’s credo of do more with less: The 45 people he oversees globally look after $2 trillion in assets. “Our assets per head are incredible,” he said. At $44 billion per person, that certainly qualifies as an understatement."