How's that index fund in your 401(k) doing? Wrong question. The right one: How much is it costing you? Our annual survey of exchange-traded funds zeroes in on the ones with the lowest overhead. If you are buying a passive index fund, cost differences are the most important thing in choosing one product over another. Past performance is irrelevant. You're not buying a foreign stock index fund or a long-term bond fund because it went up last year. You're buying it because your portfolio needs more exposure to those asset classes.
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