The 80/20 rule (or Pareto Principle (https://en.wikipedia.org/wiki/Pareto_principle)) is a phenomenon originally noted by economist Vilfredo Pareto when he found that 80% of the land in Italy was owned by %20 of the population. This concept, that there is a power law distribution (Power Law) common in natural orders, can be abstracted to many other contexts.

In general though, it's practically used to remember to put most of your focus on the parts of a system that are most important.

Examples

• Microsoft found (https://www.crn.com/news/security/18821726/microsofts-ceo-80-20-rule-applies-to-bugs-not-just-features.htm) that fixing the top 20% of most reported bugs, 80% of the errors would resolve themselves.
• 80% of software can be written in 20% of the allotted time. But the hardest 20% of code takes 80% of overall coding time. (Pressman, Roger S. (2010). Software Engineering: A Practitioner's Approach (7th ed.). Boston, Mass: McGraw-Hill, 2010. ISBN 978-0-07-337597-7. #zt)
• The top 20% of earners in the US have paid ~80% of taxes. (https://www.wsj.com/articles/top-20-of-americans-will-pay-87-of-income-tax-1523007001)

Citations

#toolong