Pareto Principle
The Pareto Principle, often called the 80/20 rule, is a simple observation: in many situations, a small number of causes are responsible for most of the results. Roughly 80% of your effects (like defects, complaints, delays, or risks) tend to come from just 20% of the causes. The exact numbers won't always be perfectly 80 and 20 — the point is that effects are usually not spread out evenly.
There's no arithmetic formula to memorize here. Instead, you gather data on a problem, group it by cause, and count how often each cause happens (or how much impact each one has). Then you sort the causes from biggest to smallest. When you add up the impact, you'll often find that a handful of causes at the top account for the lion's share of the trouble.
Reading the result is intuitive: if a few causes are creating most of your problems, those are the ones to fix first. A project manager (the person responsible for planning and delivering a project) uses this to focus limited time and money where it will do the most good — instead of trying to fix everything at once, you tackle the 'vital few' causes that drive the most pain.
Think of it like tidying a messy room where clothes are scattered everywhere. If you pick up the few big piles first — the jacket, the towels, the gym bag — the room instantly looks 80% cleaner, even though you've only touched a small share of the items on the floor.
Imagine a software team logs 100 customer complaints over a month. When you sort the complaints by cause, you find: slow login (46 complaints), payment errors (28), confusing menus (12), and then 14 complaints spread across 9 other minor issues. Just 2 causes — slow login and payment errors — account for 74 of the 100 complaints, about 74%. Following the Pareto Principle, the project manager decides to fix those two issues first, because solving them removes nearly three-quarters of all the complaints with only a fraction of the effort.
Every PMP formula explained free — plus worked examples and practice in PMP Math, and full timed mocks in the simulator.