Glossary term
Incidence
The rate at which new cases of a condition appear in a population over a period of time.
Visual intuition
same population
+2 cases
Incidence: the 2 new cases this year
The same population of 100 watched over a year: 2 people already had the condition, and 2 new coral cases appeared — an incidence of 2 per 100 per year (illustrative counts).
Formula
incidence = new cases in the period / people at risk
Example
If 200 of a city's 100,000 residents are diagnosed with flu during one winter month, the monthly incidence is 0.2% — even if far more are still recovering from earlier infections (illustrative numbers).
How It Works
Watch a over a period — a year, say — and count who newly develops the condition: 2 new cases among 100 people is an incidence of 2 per 100 per year. Incidence is a flow where is a stock, and they can move in opposite directions: a treatment that keeps people alive longer changes nothing about new cases but raises prevalence, while a vaccine lowers incidence long before prevalence follows it down.
Concepts
Base Rates
The background frequency of something before you consider new evidence. A strong signal can still produce many false alarms when the thing being tested for is rare, because there are many more non-cases than cases. Good reasoning combines the new signal with the base rate.
Comparing Risks
Putting different dangers on one common scale — like the micromort, a one-in-a-million chance of death — lets you compare activities honestly, instead of ranking them by how frightening they feel.
