Exchange-traded funds have been nearly synonymous with passive investing ever since the industry was launched in the 1990s.

Unlike mutual funds — which are often actively managed and traded once a day after the market close — ETFs are traded throughout the day, like stocks. They tend to be passively managed, mirroring an index or sector. That helps ETFs maintain one of their supposed advantages over mutual funds: They are cheaper, with fees well below 1 percent. Mutual fund fees are often twice as high.

A growing number of ETF sponsors are adding actively managed funds to their offerings, though. While a certain number of actively managed ETFs have been around for years, their ranks are on the rise. Russell, whose indexes are the basis for many ETFs, said last year it would launch six actively managed ETFs. It acquired the actively managed U.S. One Fund to launch its expansion. Powershares, AdvisorShares, and Vanguard have actively managed ETFs too.....

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