Epidemiology: Everything Turns on One Number Crossing One
If each case produces more than one further case, an outbreak grows; below one, it fades. That threshold is simple — and almost everything else about the subject is context-dependent.
Topic
Epidemiology & Disease Dynamics
The Central Limit Theorem: Why Bell Curves Are Everywhere
One die is flat. Ten dice added together make a bell curve. The shape is a property of adding independent things — which tells you exactly when to expect it and when not to.
The Law of Large Numbers: Small Samples Are Wild
Seven heads in ten flips is unremarkable; seventy thousand in a hundred thousand would be extraordinary. The same probability, different room for chance — with consequences most people miss.
Simpson's Paradox: True in Every Group, False Overall
A treatment beats its rival for small stones, beats it for large stones, and loses when you combine the patients. Nothing was miscalculated — which is what makes it worth understanding.
Longevity and Health: The Big Effects Are the Ones Nobody Sells
The interventions with the largest effects have been known for decades and are unglamorous. Attention flows almost inversely to effect size, for a simple reason — the big ones cannot be sold.
The Base Rate Fallacy: Start With How Common It Is
The quiet, bookish stranger sounds like a librarian. There are a hundred times more salespeople — so the shy ones alone outnumber every librarian in the country.
Bayes' Theorem: The 99% Accurate Test That Is Wrong 91% of the Time
A rare disease, a 99% accurate test, a positive result — and roughly a 9% chance you're ill. The gap comes from one number most people leave out entirely.
The Red Queen Hypothesis: Running Hard to Stay Exactly Where You Are
Species that have survived for ten million years go extinct at about the same rate as new ones. That surprising fact explains why your improvements so often buy you nothing.