Definition
The 80/20 rule — also called the Pareto principle — is the empirical observation that, in many distributions of contributors to a total, the top 20% of items account for roughly 80% of the total. The exact split varies, but the long-tail shape is remarkably common.
Rosling uses it as a triage heuristic: when handed a list of 200 things, find the top items, deal with those first, and ignore the rest until the top ones are handled.
Why it matters
How it works
Sort the list of contributors descending by size of contribution. Walk down until you reach the row where cumulative share crosses 80%. Those rows above the line are your priority set; the rest are the long tail.
The 80/20 ratio is a shorthand for a power-law-ish distribution, not a law of nature. Real distributions of word frequency, file sizes, city populations, or cause-of-death rates often follow steeper or shallower curves — 90/10, 70/30, or genuinely uniform. The discipline is to look at the actual sort, not assume the canonical numbers.
For Rosling's purposes the rule mostly cures the Size Instinct: when a single big number arrives without comparators, ask whether it is one of the top-20% items or a long-tail distraction. If it is long-tail, attention elsewhere will pay off more even if the headline is impressive.