An equal S&P would be down for the year

Trader on the floor of the New York Stock Exchange.
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Trader on the floor of the New York Stock Exchange.

The stock sell-off in the past two days has investors worried that the market will give back its October gains and end the year in the red.

More concerning to some analysts is that strength in the market seems to be concentrated in just a few companies. The S&P 500, for example, has seen its growth this year mainly in large cap companies. The narrow market breadth is unusual in the past few years.

"The 10 largest stocks in the S&P 500 have contributed more than 100% of the year's roughly 2% gain," researchers from Strategas Research Partners wrote in a note, according to USA Today. "By comparison, both 2013 and 2014 saw the 10 largest stocks contribute less than 20% of the year's advance."

Because the S&P 500 is market-cap weighted, big companies have a greater influence on the direction of the index. If the stocks were given equal weight, the index would be down from the beginning of 2015.

Large cap S&P 500 companies have risen an average of 0.8 percent since January, while the rest of the index has given back 9.6 percent, according to data from FactSet.

Technology stocks have led the index in year to date. Netflix and Amazon have increased more than 100 percent as of market close Monday. Gaming company Activision Blizzard — which recently announced its $5.9 billion takeover of mobile gamer King Digital — is up nearly 70 percent from January.