An Italian economist, Vilfredo Pareto, noticed that 80% of Italy's land was owned by 20% of the population. This observation spawned the Pareto Principle, that "for many events, roughly 80% of the effects come from 20% of the causes".
In other words, 80% of the sales probably come from 20% of the customers, 80% of the hits to a site probably come from 20% of the pages, etc.
The personal experience of this author is that 80% of business decisions are made by following the Pareto Principle backwards -- by putting concentration on the 80% half that has no impact instead of the 20% that has an impact -- and thus we get into discussions where outliers as being part of "the 20%" are discounted when they shouldn't be. This is especially true for usability/accessibility issues, where 80% of the usability issues are egregious for 20% of the users... but 100% of the users benefit from the corrections. Instead of correcting the issue for the 20% who can't use the site, we often hear an argument that we should concentrate on the 80% of the users not experiencing severe issues.