80-20-rule

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# The 80/20 Rule

The 80/20 rule (also called the Pareto principle, after Italian economist Vilfredo Pareto) is the empirical observation that, in many systems, a small fraction of inputs produces most of the outputs — roughly 80% of effects from 20% of causes. The exact numbers vary (sometimes 70/30, sometimes 90/10) but the shape — a heavily uneven distribution — recurs across economics, management, software usage, and design.

For designers, the principle's central use is *prioritization*: identify the 20% of features or content that produces 80% of the value, design that 20% well, and let the remaining 80% receive proportionally less effort.

## Definition (in our own words)

In most large systems, effects aren't distributed evenly across causes. A small minority of features account for the bulk of usage. A small minority of users generate most of the support tickets. A small minority of bugs cause most of the crashes. A small minority of tasks consume most of the time. The 80/20 rule names the pattern and suggests that designers should put their effort where it produces the most return: the critical 20%, not the trailing 80%.

## Origins and research lineage

- **Vilfredo Pareto** (1848–1923), Italian economist. Originally observed that approximately 20% of Italians owned 80% of the land. The pattern proved remarkably general.
- **Joseph M. Juran** (1951), in *Quality Control Handbook*. Generalized the pattern from economics to industrial quality control: a small fraction of defect causes accounted for most defects. Juran called this "the vital few and the trivial many," a phrasing still used.