The money keeps pouring into mutual funds: Investors pumped another $30 billion into stock, bond, and hybrid funds in May, according to the Investment Company Institute, the industry's main trade group. A figure that's less publicized than new sales but is equally important is redemptions. Even as one investor buys new shares in a fund, another may be cashing out. The difference, known as net sales or net flows, is the key to fund companies' profits.

"The industry doesn't focus very hard on the retention side," said Geoffrey Bobroff, a mutual fund consultant in E. Greenwich, R.I. "But revenues to advisers are often based on net flows. That's the important number. "

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