From Morningstar:

Adding Amazon to the Dow will change the index’s dynamics. The Dow calculates prices based on stock prices, not market caps like other indices. Walmart’s stock split and Amazon’s high price will impact the index. The Dow seeks more exposure to consumer retail with Amazon’s inclusion. Changes can affect stock performance – for example, Apple outperformed AT&T after joining. The Dow’s changes aim to match the market, though new stocks can underperform. The correlation between Dow stocks and the S&P 500 has been strong over time. Watch how the Dow reacts to Amazon’s addition and other changes.



Read more at Morningstar: Amazon’s stock could lose to Walgreens’ this year if the Dow jinx holds