Traders at the New York Stock Exchange. (Photo: AP) Traders at the New York Stock Exchange. (Photo: AP)

Want to know how seriously the stock market is concerned about a trade war between the U.S. and China? Watch the Dow Jones industrial average.

This concentrated, price-weighted index of 30 stocks is dominated by companies that either sell into global markets or source materials outside the U.S. It is a a better gauge of the potential impact of the U.S.-China tariff dispute than the more diversified S&P 500.

(Related: The Stock Market Doesn’t Want a Trade War)

Year to date through Friday, the Dow is down 3.18% while the S&P 500 is off 2.59%. When markets plummeted Friday after President Donald Trump threatened U.S. tariffs on an additional $100 billion worth of Chinese imports, the Dow fell 2.34%; the S&P 500 dropped  2.19%.

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“Market fears over multilateral trade wars have dinged the Dow much more than other market measures,” writes Nicholas Colas, co-founder of DataTrek Research, in a recent market analysis.

He notes that five of the Dow’s 30 companies have global supply chains or global sales footprints: 3M, Caterpillar, Home Depot, P&G and Walmart. Home Depot and Walmart buy source materials globally; 3M, Procter & Gamble sell products globally and Caterpillar does both.

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“Looking through the Dow’s weightings, there aren’t many paths available for the Dow to go up on the year without some help from these stocks,” writes Colas.

The five stocks constitute 20% of the weight of the Dow, which is a price-weighted index, unlike the S&P 500, which is capitalization weighted. In the Dow, the more expensive a company’s stock price, the heavier its weight in the index.

Many commentaries on a potential trade war between the U.S. and China focus on Boeing, which accounts for more than 9% of the Dow. Boeing is the biggest exporter among U.S. companies, with reportedly over $1 trillion in potential sales over the next 20 years and accounts for more than half the commercial airliners operating in China. And China has identified aerospace companies as a target of proposed 25% tariff. But Boeing, Colas says, “is actually holding up the Dow in 2018 not pulling it lower.”

(Related: Trade Wars and Protectionism Unsettle Institutional Investors)

Three of the other eight Dow gainers so far this year are tech stocks: Cisco, Intel and Microsoft; and three are financial stocks: JPMorgan, Travelers and Visa.

DataTrek doesn’t expect a recovery in the Dow until worries over trade talks abates. One indicator of that: When the Dow beats the S&P 500. “The average, while an ancient market measure, really is the barometer to watch on this important market dynamic,” writes Colas.