The Dow Jones Industrial Average has been outperforming the S&P 500 this year thanks in part to a trio of high-flying stocks at the top of the Dow and a relatively lower weighting in financial stocks, a market strategist said Friday.

The Dow has truly been the “rock star” large-cap stock index of 2011, said Nicholas Colas, chief market strategist at ConvergEx Group.

SPDR Dow Jones Industrial Average ETF (NYSEArca: DIA) was up 11.1% year to date as of July 21, compared with a 7.9% gain for iShares S&P 500 (NYSEArca: IVV).

“That outperformance comes from a trio of stocks — IBM (NYSE: IBM), Caterpillar (NYSE: CAT) and Chevron (NYSE: CVX) — with large weightings in the Dow and great year-to-date performance,” Colas said.

However, Caterpillar shares slipped 6% on Friday after the company’s earnings missed expectations. [Industrials ETFs in Focus on Caterpillar, GE Earnings]

The Dow and S&P 500 have key differences. The Dow is a “price-weighted” index, meaning that stocks with high share prices account for a larger portion of the index. The S&P 500, meanwhile, weights stocks by their market capitalization, or the share price multiplied by the number of shares outstanding.

IBM, Caterpillar and Chevron are the three largest holdings in the Dow ETF at 11%, 6.6% and 6.5%, respectively.

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