The 80/20 Rule

The 80-20 rule, also known as the Pareto Principle, is an aphorism which asserts that 80% of outcomes (or outputs) result from 20% of all causes (or inputs) for any given event. In business, a goal of the 80-20 rule is to identify inputs that are potentially the most productive and make them the priority.

-- ^{From a Google Search: The 80-20 rule}

See UHFD & Eli Klein