80/20 rule

(or Pareto Principle)

Roughly 80% of the effects come from 20% of the causes. This applies to many events observed in the world of economy, biology, computing, sport and many other areas.

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It also might mean that you can identify the 20% of work that will bring you 80% of outcomes. However, often to get to from 80% to 100% of outcomes requires 80% of effort (think learning a language, wrapping a complicated project, becoming a master in some areas etc.)