AGNC Investment has a towering dividend yield, but don't pin your retirement dreams on this mortgage REIT.Investors have a bad habit of getting fixated on key statistics when examining stocks, often using one simple metric when a multivariate approach would be better. For example, AGNC Investment's (AGNC 0.92%) huge 14%-plus dividend yield might make income investors think they found a millionaire-maker stock. But digging in just a little will show that this isn't likely to be the case. Here's what you need ...