"Portfolio management is a balancing act," says iShares Portfolio Manager Dina Ting. "Over the long term, we've delivered on our objectives."

Executives at other ETF firms echoed iShares in saying investors can't necessarily expect the tracking error for all types of funds to be as small as it is for ETFs that target just large U.S. stocks. In the recent junk-bond rally, for instance, some of the hardest-to-trade securities performed the best, notes State Street. These bonds are much harder to buy than stocks of well-known companies. Telecommunications funds have trouble tracking that industry because diversification rules designed to protect investors limit their holdings of giant players like AT&T Inc. and Verizon Communications Inc., says Vanguard.

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