Stocks extended their losses for a second day Thursday, dragged by health care and techs, after a batch of economic data that missed projections and mixed earnings reports.
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Stocks gyrated in a wide range this week, with all three major benchmark indexes suffering their sharpest one-day drop of the year on Monday, before recovering most of those losses on Tuesday. However, after Wednesday and Thursday's consecutive declines, stocks are on track for their worst weekly losses of the year.
"You can tell equity investors are on edge and looking for a reason, almost any reason, to take a little money off the table," wrote Scott Wren, senior equity strategist at Wells Fargo Advisors. "A series of worse than expected economic reports out of China, Europe and the U.S. over the last handful of trading days has taken some of the enthusiasm out of the market."
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The Dow Jones Industrial Average slumped 81.45 points to close at 14,537.14, dragged by UnitedHealth and Bank of America, while Verizon gained.
The S&P 500 declined 10.40 points, to finish at 1,541.61, finishing below its 50-day moving average of 1,543 for the first time this year, signaling the market's recent uptrend could be losing momentum. The Nasdaq dropped 38.31 points, to end at 3,166.36.
Adding to woes, the Nasdaq 100 and the Russell 2000 indexes both closed below their 50-day averages this week.
The CBOE Volatility Index (VIX), widely considered the best gauge of fear in the market, jumped above 17.
Among key S&P sectors, health care and techs led the decliners, while telecoms gained.
Apple extended its decline from the previous session to close below $400 a share for the first time since December 2011.
With Apple's recent losses, oil company Exxon Mobil recaptured its title as the world's most valuable company in terms of market cap.
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Among earnings, Morgan Stanley reported quarterly results that topped expectations, thanks to strong gains in its global wealth management and securities units. However, revenue from fixed income and commodities trading fell from a year earlier, reflecting declines in commodities and rates. Shares declined, also dragging other major banks including Bank of America and Goldman Sachs.
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PepsiCo advanced after the beverage company also posted better-than-expected quarterly earnings and revenue. The company also stood by its full-year forecast. Also, Verizon gained after the telecommunications company topped earnings estimates, but revenue fell slightly below expectations.
Peabody Energy soared after the coal miner posted a smaller-than-expected loss, but revenue fell just shy of analysts' expectations.
Meanwhile, UnitedHealth edged past earnings expectations, but shares declined sharply as the health care company said that a major public-sector customer had switched to a fee-based insurance service from a full-risk plan in the first quarter and lowered its 2013 sales guidance. Rivals Aetna, Humana and Wellpoint also traded lower.
SeaWorld Entertainment priced its IPO at $27 a share, at the top of its expected range. The company plans to list its shares on the New York Stock Exchange under the symbol "SEAS."
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On the economic front, weekly jobless claims gained 4,000 to a seasonally adjusted 352,000, according to the Labor Department. A Labor Department analyst said claims for California and Kentucky had been estimated.Economists polled by Reuters expected a reading of 350,000 new claims, up from 346,000 in the previous week.
Also on the economic front, business activity in the mid-Atlantic region expanded at a smaller-than-expected rate, according to the Philadelphia Fed survey. And a gauge of future U.S. economic activity dipped in March for the first time in seven months, according to the Conference Board.
"We are sticking with our long-held estimate that American economy will grow a modest 2.5 percent this year," according to Wren. "We continue to view pullbacks in the stock market as opportunities...right now some of the domestic and international data suggest that we may be experiencing a bout of turbulence, [but] the S&P 500 will likely end the year higher than current levels."
—By CNBC's JeeYeon Park (Follow JeeYeon on Twitter: @JeeYeonParkCNBC)
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