Amazon joins the Dow Monday, giving the 127-year old index a needed upgrade as it trails the S&P 500

Traders work at the New York Stock Exchange (NYSE) in New York City. The Dow Jones Industrial Average is undergoing a shakeup, with Amazon replacing Walgreens Boots Alliance. Amazon’s strong performance in the past year has propelled the S&P 500 index to new highs, while the Dow has struggled to keep up. The Dow’s smaller size and its weighting by individual stock prices have led some investors to see it as an inadequate representation of the US stock market. With the addition of Amazon, the gap between the Dow and the S&P will narrow.