Pareto Principle (80/20 Rule)
The Pareto Principle states that roughly 80% of effects come from 20% of causes. Originally observed by economist Vilfredo Pareto in 1896, who noted that 80% of Italy's land was owned by 20% of the population, the principle has since been documented across domains from business revenue (80% from 20% of customers) to software bugs (80% caused by 20% of code). As a decision-making tool, it directs attention and resources to the high-leverage minority of inputs rather than distributing effort uniformly.