Election forecasting superhero Nate Silver explains what it takes to make a good forecast in The Signal and the Noise
The key to making a good forecast … is not in limiting yourself to quantitative information. Rather, it's having a good process for weighing the information appropriately. … collect as much information as possible, but then be as rigorous and disciplined as possible when analyzing it.
Silver makes a great point a few pages after this quote writing: “… when we have trouble categorizing something, we'll often overlook it or misjudge it.”