Ever since I learned it, I've loved the Pareto principle, the 80/20 thing. I particularly applied it to software, most notably back in the day when we were trying to automate deficiency payments. I think I can claim credit, or blame, for splitting payments into two categories: special and regular. The regular ones we'd try to run as a batch, the special we'd struggle with as best we could.
Now people have discovered the 80/20 rule works for sports, specifically in things like tennis the best 20 percent of the players win 80 percent of the prizes. See this Technology Review piece