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Image source: 2015 New Year celebration by christmasstockimages.com.

The Dow Jones Industrials (DJINDICES:^DJI) had a forgettable year in 2015, with the average posting a 2% decline to finish at 17,425 and losing ground for the first time since the financial crisis in 2008. Yet even though the Dow's performance was an obvious disappointment for investors, a number of interesting things happened with the Dow over the course of the year that will have longer-term implications for stock market investors. Let's look more closely at some of the most important things that happened to the Dow in 2015.

Apple joins the Dow
Perhaps the most awaited event for the Dow in 2015 was the admittance of Apple (NASDAQ:AAPL) to the average in March. For years, the tech giant's high share price was a barrier to its entry to the price-weighted Dow, as it would have had far too much influence over the average. Apple's decision to split its stock 7-for-1 opened the door for the stock to come into the Dow without having too heavy a weighting in its calculation.

Apple didn't have the positive impact that most had probably hoped to see, with the stock falling for the first time in years. Indeed, as we've seen in the past, the stock that got taken out of the Dow to make room for Apple, AT&T, actually performed better, climbing about 10% on the year. Nevertheless, as the largest company in the stock market, Apple deserves its place in the Dow, whether or not it has a positive impact on returns in the future.

Dow has first correction since 2011
On average, the Dow has a 10% downward correction about once a year, but for three years, investors had gone without a loss of that magnitude. That all changed in 2015, when U.S. stocks responded negatively to a 40% plunge in the Chinese stock market over a period of just a couple of months during the summer. Concerns about a potential spillover to the global economy ran rampant, and at its worst point in August, the Dow traded more than 16% off its record levels from earlier in the year.

The correction fell short of the 20% downward move that defines a bear market, so, technically, the bull market remains intact as it approaches its seventh anniversary. While many saw increased volatility as something to fear, experienced investors were somewhat pleased to see an unprecedented level of calm finally come to an end and return to more normal conditions.

Big winners and losers
Finally, it's always tempting to think that a year near the unchanged level must have been inconsequential for every stock in the Dow. Yet underneath that flat performance were some big winners and losers.

On the plus side, consumer giants Nike (NYSE:NKE) and McDonald's (NYSE:MCD) were the biggest gainers, both climbing about 32% to 33%. For Nike, the positive trend toward athletic apparel and footwear continued, and even as the stalwart faces competition from newer companies in the space, Nike has managed to consolidate its leadership position in the industry by sticking to its long-term business model and continuing to innovate. For McDonald's, a much-needed recovery came from a willingness to try new things, including the long-awaited introduction of all-day breakfast to its menu. With the fast-food giant looking for ways to simplify its business for its franchisees, McDonald's hopes to return to its roots and get customers reconnected to the restaurant chain's food offerings.

Leading the losers in the Dow were Wal-Mart (NYSE:WMT) and American Express (NYSE:AXP), which fell 26% and 24%, respectively. Wal-Mart said late in the year that the need to spend more on labor costs and spending to further develop its e-commerce platform would lead to an earnings decline of as much as 12% in the coming fiscal year. The news further weakened confidence in Wal-Mart, which has struggled in recent years to keep its sales moving higher. For American Express, the loss of a key merchant card relationship with Costco Wholesale put pressure on the financial giant, which has seen competition get ever fiercer with its rival card networks. American Express hopes to earn back business elsewhere to make up for the loss, but shareholders aren't certain whether it will be successful in keeping up with its biggest competitors.

2015 was a disappointing year for Dow Jones Industrials investors, but it serves as a reminder that declines do happen from time to time. The admission of Apple to the Dow was a key event that could drive future returns for the average, and the appearance of a correction was something that experienced investors had wanted to see in order to dispel fears of unsustainable gains.

Dan Caplinger owns shares of Apple. The Motley Fool owns shares of and recommends Apple and Nike. The Motley Fool recommends American Express. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.