Universal Principles of Design, Updated and Expanded Third Edition

133 Pareto Principle

A small percentage of variables in any large system is responsible for most of its behaviors.

The Pareto principle, proposed by the economist Vilfredo Pareto, states that approximately 80% of the dynamics of any large system is caused by 20% of the variables comprising that system. The Pareto principle is observed in all large, complex systems — i.e., systems that involve many independent variables interacting with one another in different ways — including those in economics, management, product design, quality control, and engineering, to name a few. For example, 20% of a product’s features are used 80% of the time; 20% of a codebase is responsible for 80% of its bugs; 20% of a town’s roads host 80% of its traffic.1