Guess and Check

When a doctor says something like "You only have about a month to live," how do they calculate that exactly? Is it just a guess or from experience, or maybe they have a Facebook quiz-like thing that spits out a number when they bubble in my information? I marked the date on the calendar, but I think you, your mother and I should go down to the Ruby Tuesday the night before, just in case. Anyway, this isn't a very good bedtime story, is it.