
Mental Model
Pareto Principle (or 80/20 Principle)
Summary
The 80/20 principle is a type of power law which states that roughly 20% of a set of causes result in 80% of the effects.
The numbers don't have to be exactly 80/20 though, and they don't have to add up to 100%: both '25/95' and '5/70' are also valid.
The key idea is that in most cases cause and effect are not linear: a small subset of causes usual generate most of the effects.