Free · 10 questions · 5 min · no sign-up
Probability test
Probability is where intuition fails most reliably. Streaks, coincidences, medical tests and game shows all have answers that feel wrong until you see why they’re right.
A fair coin lands heads five times in a row. What’s the chance the next flip is heads?
About this test
Every wrong answer here is a named mistake. You’ll find out after each question, and at the end you’ll see which mistakes caught you, with a course that starts from exactly those.
Questions people ask
What’s the most common probability mistake?
Ignoring the base rate, especially with medical tests: a positive result from an accurate test for a rare condition is often more likely to be a false alarm than a real case.
Is the Monty Hall problem really 2 in 3?
Yes. Your first pick is right 1 time in 3. The host always reveals an empty door, so switching wins whenever your first pick was wrong: 2 times in 3.
Does this need maths?
Only a little arithmetic. Every question can be reasoned out by picturing a crowd of people or a lot of coin flips.
Read more
- Base Rate Neglect: Why a Positive Test Often Isn't What It Seems
- The Monty Hall Problem, Explained So It Finally Clicks
- The Gambler's Fallacy: Is Tails Ever "Due"?
- The Birthday Paradox: Why 23 People Is Enough
- Relative vs Absolute Risk: What "50% Higher Risk" Really Means
- The Law of Large Numbers, Explained With Coin Flips