The Pareto principle: 80% of the effects come from 20% of the effort. Really quite a lot of things show a power law.

Ferriss puts it like this:

The 80/20 principle, also known as Pareto’s Law, dictates that 80% of your desired outcomes are the result of 20% of your activities or inputs. Once per week, stop putting out fires for an afternoon and run the numbers to ensure you’re placing effort in high-yield areas: What 20% of customers/products/regions are producing 80% of the profit? What are the factors that could account for this?

He considers this a useful principle to apply to everything. And it is - I don't necessarily throw out the unproductive 80% on a given measure (I might want it for other reasons), but it is interesting to see if there's a ready win there. And it's useful even when you work an ordinary salaried day job, as I do. (e.g. these two weeks, when my boss is on holiday and I'm doing all his job as well as my own.)