Then as portfolio managers sold stocks to lock in gains during the last week of that March, the QQQ, which is the ticker symbol the ETF that consists of all of the stocks in the Nasdaq-100 Index, dropped 15 percent in just one week.

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Vendors with app managers include Net Nanny, App Lock, and Smart AppLock.

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Most of these are actively managed funds where managers attempt to outperform a benchmark by selling winners to lock in gains.

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