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Just 10 stocks are having an incredible impact on the market this year

Digging into whether the market is too top-heavy

The most productive stocks appear to be doing more than their fair share this year.

According to Barclays U.S. equity strategist Keith Parker, just 10 stocks — predominantly technology names — have accounted for 41 percent of the S&P 500's 8 percent return so far this year.

This contribution from the top 10 stocks is 11 percentage points above average levels, according to Parker, who sees opportunities to reallocate out of growth stocks and "selectively buy 'value' laggards."

"It's not unusual for five to 10 stocks to drive a significant part of the return, i.e., 30 percent, but to have so much through May, I think was the outlier," Parker said Friday on CNBC's "Trading Nation."

Parker's call about these particular stocks — Apple, Amazon, Facebook, Microsoft, Johnson & Johnson, two classes of Google shares, Philip Morris, Visa and Comcast (the parent company of NBCUniversal and CNBC) — was actually rather prophetic, as he published his report a little over a week prior to a sell-off within the technology sector earlier this month.

In his May 31 note, he wrote, "positioning is now stretched for growth and tech stocks."

Furthermore, this has now caused the market to appear more "top-heavy," with respect to these contributions, than it has been since 2009. The rest of the market, as Parker pointed out, has moved very little relative to the movement within these 10 stocks. In fact, when his note was released, 10 names were responsible for almost half of the S&P's year-to-date gain.

Earlier this month, big technology stocks declined amid an apparent market rotation. Still, the information technology sector remains the market's leader this year.

For retail investors at this juncture, Parker would recommend selectively buying "value" stocks such as financials and industrials, which have lagged the broader market, and reducing holdings within technology and consumer discretionary.

At the "single stock level," he said Friday, "a lot of the fiscal upside and tax has been priced out of the market on a single stock basis, and so positioning for that potentially changing and shifting over the coming months and quarter" would be wise.

Is the market too top-heavy?