Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The stock market largely continued its post-new year's hangover Friday, as the S&P 500 and Nasdaq Composite both fell, even though the Dow Jones Industrials managed to push higher. Helping to weigh on sentiment today were shares of International (TCOM -2.38%), Arch Coal (NYSE: ACI), and Inovio Pharmaceuticals (INO 2.37%), all of which suffered more substantial declines.

Ctrip dropped 8% as a Bloomberg report detailed the heightened competition that the online travel company could see in the near future. With companies like newly public Qunar Cayman Islands saying that it saw record levels for bookings of air travel in late 2013, Ctrip is suddenly facing threats not just on the hotel side of its business, but also on airline ticketing. Yet, Ctrip still has its size advantage to use, and U.S. giant (BKNG 0.07%) has demonstrated the value of using its size to create other competitive advantages it has used to send its shares soaring.

Arch Coal fell 5%, as most of its peers in the coal sector suffered a bad day of losses. The problem that Arch and its competitors all face is that coal demand has been weak for years, especially in the U.S. with its relative glut of low-cost natural gas. As utilities and other high-volume coal users have shifted to natural gas, it has taken away a big source of revenue for Arch and other coal producers. Arch Coal has done its best to boost its exports to reach areas of the world where coal is in higher demand, such as India and China. But if U.S. buyers never come back into the coal market, Arch Coal could see further declines, despite its already substantial losses last year.

Inovio Pharmaceuticals plummeted 13%, but in the context of last year's huge gains for the biotech's stock, the decline marked only a modest correction. In 2013, Inovio's shares almost quadrupled, as the developer of cancer immunotherapy treatments teamed up with major partner Roche to boost its odds of getting its pipeline prospects to market. Yet, with Inovio's pipeline filled with early-stage candidates, the company still has to prove itself with successful results. Until it does, the stock will be vulnerable to potentially wild swings both up and down that match the mood of investors on any given day.