If you're a fan of Cosmos or just an astronomy buff in general, then you've surely heard of the Drake Equation. Formulated in 1961 by Dr. Frank Drake, duly pictured here, it's a mathematical equation designed to predict the number of possible extra-terrestrial civilizations out there. It's fairly straightforward. First, figure out the average rate that stars are born. Next, figure what fraction of those might have planets. Now figure how many of those planets can support life. Next, how many of them do support life, and so on. Follow this pattern far enough and eventually the formula tells us how many Frank Drakes there might be in the universe.
Whatever number is produced by the formula is essentially meaningless, for many of the equation's factors are based on pure conjecture. But the general principle is sound: start with a large pool and whittle it down until you have the finalists.
If you think about it, variations of this formula can be created and applied to almost anything. Take, for example, the one hundred thousand people who audition for American Idol each year. A small fraction of those make it past the scouts. A small fraction of those make it past the producers. A small fraction of those actually make it in front of the judges. Another fraction goes to Hollywood, and so on. Eventually, you get down to the season finale where one person, usually Frank Drake, is declared the winner.