One of the biggest questions stock investors have is when to sell. To shed some light on the subject, we sat down with Matt Coffina, an equity strategist at Morningstar and editor of the Morningstar StockInvestor newsletter.

Question: Matt, what are the key things that investors should keep in mind before exiting a stock position? Is a big sell-off or runup in the stock enough to raise a red flag indicating that you should get out?

Coffina: The key thing to keep in mind is how a stock's price compares to its intrinsic value. This is true both on the upside and the downside. Say some news comes out that a company's margins are going to be much higher than other investors previously anticipated, than you previously anticipated, and the stock runs up 20%. Is that a time to sell? Well maybe the fair value estimate on that stock is actually up 30%, in which case it could be an even bigger bargain than it was before the news was released.