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.

ECONOMIST: The risks created by complicating a simple idea

Michael Kaback, emeritus professor of pediatrics and reproductive medicine at the University of California, San Diego School of Medicine, says trying to determine the frequency of carriers for Tay-Sachs among the Irish is likely to be complex.

WSJ: New Study Looks at Irish Risk for Tay-Sachs Disease