Fall 2026 · Week 01
A Quick Quiz
No lecture today.
Three rules
- No phones.
- Being wrong is not embarrassing.
- Being wrong and confident is.
Every answer comes with a number
Pick A, B, or C. Then write how sure you are.
- 50% — a pure guess
- 70% — fairly sure
- 90% — almost certain
1 / 13
In all low-income countries across the world today, how many girls finish primary school?
- A20%
- B40%
- C60%
How sure are you?50%70%90%
2 / 13
Where does the majority of the world population live?
- ALow-income countries
- BMiddle-income countries
- CHigh-income countries
How sure are you?50%70%90%
3 / 13
In the last 20 years, the proportion of the world population living in extreme poverty has…
- AAlmost doubled
- BStayed more or less the same
- CAlmost halved
How sure are you?50%70%90%
4 / 13
What is the life expectancy of the world today?
- A50 years
- B60 years
- C70 years
How sure are you?50%70%90%
5 / 13
There are 2 billion children in the world today, aged 0 to 15. How many children will there be in 2100, according to the United Nations?
- A4 billion
- B3 billion
- C2 billion
How sure are you?50%70%90%
6 / 13
The UN predicts that by 2100 the world population will have grown by another 4 billion people. What is the main reason?
- AMore children (under 15)
- BMore adults (15 to 74)
- CMore very old people (75+)
How sure are you?50%70%90%
7 / 13
How did the number of deaths per year from natural disasters change over the last hundred years?
- AMore than doubled
- BStayed about the same
- CFell to less than half
How sure are you?50%70%90%
8 / 13
About 7 billion people live in the world today. Which split is closest to where they live? (Americas / Europe / Africa / Asia, in billions)
- A1 / 1 / 1 / 4
- B1 / 1 / 2 / 3
- C2 / 1 / 1 / 3
How sure are you?50%70%90%
9 / 13
How many of the world’s one-year-old children have been vaccinated against at least one disease?
- A20%
- B50%
- C80%
How sure are you?50%70%90%
10 / 13
Worldwide, 30-year-old men have spent 10 years in school on average. How many years have women of the same age spent in school?
- A9 years
- B6 years
- C3 years
How sure are you?50%70%90%
11 / 13
In 1996, tigers, giant pandas, and black rhinos were all listed as endangered. How many of these three are more critically endangered today?
- ATwo of them
- BOne of them
- CNone of them
How sure are you?50%70%90%
12 / 13
How many people in the world have some access to electricity?
- A20%
- B50%
- C80%
How sure are you?50%70%90%
13 / 13
Global climate experts believe that over the next 100 years the average temperature will…
- AGet warmer
- BStay the same
- CGet colder
How sure are you?50%70%90%
Mark your own card
- 1 – 4
- C · B · C · C
- 5 – 8
- C · B · C · A
- 9 – 13
- C · A · C · C · A
Questions 1 – 12
How did we do?
The six of us, on average: __ / 12
A chimpanzee, guessing: 4 / 12
Tens of thousands of adults: 2 / 12Question 13 is left out: it is the only one humans win.
The questions you were "almost certain" about
Marked 90%: __ questions Actually correct: __ Hit rate: __ %
Usually thirty to forty percent.
Two names
Overconfidence, and its opposite
A weather forecaster who says "70% chance of rain" is well calibrated if, across all the days she says 70%, it rains on about seven in ten.
She does not need to be right every day. She needs her 70% to mean 70%.
The goal from today: when you say 90%, be right nine times in ten.
Adults are no better
Researchers gave thousands of business managers the same kind of test. Answers they marked "90% sure" were right forty to sixty percent of the time.
It is not that you are young. It is that nobody practises this.
Look at the direction of your mistakes
Random guessing scatters errors in every direction. Yours almost all point the same way: "the world is worse than it is."
So it was not luck answering for you. It was a ready-made picture of the world already in your head. Where did that picture come from? Hold that question; Week 3 answers it.
One more thing
AI never tells you how sure it is
Every sentence it writes sounds like 90%. In Week 7 we will make it show its confidence. You learn first.
Next: let us measure what your "90%" is actually worth.
Round 1 · on your own · 10 questions
Give a range, not a number
For each question write a low end and a high end. You should be 90% sure the true answer lies between them.
- Only two things count: did the range contain the answer, and, if so, how narrow it was.
- A wide range is not embarrassing. Saying 90% and missing is.
- One minute per question.
1 / 10 · 60 s
Average distance from the Earth to the Moon, in kilometres
–
Write a range you are 90% sure contains the answer.
2 / 10 · 60 s
How many languages are spoken in the world
–
Write a range you are 90% sure contains the answer.
3 / 10 · 60 s
Litres of blood in an adult human body
–
Write a range you are 90% sure contains the answer.
4 / 10 · 60 s
Number of seconds in one day
–
Write a range you are 90% sure contains the answer.
5 / 10 · 60 s
Frames in a two-hour film at 24 frames per second
–
Write a range you are 90% sure contains the answer.
6 / 10 · 60 s
Heartbeats in a 75-year life
–
Write a range you are 90% sure contains the answer.
7 / 10 · 60 s
Drops of water in one litre
–
Write a range you are 90% sure contains the answer.
8 / 10 · 60 s
Population of China
–
Write a range you are 90% sure contains the answer.
9 / 10 · 60 s
Population of North Carolina
–
Write a range you are 90% sure contains the answer.
10 / 10 · 60 s
Population of Cary
–
Write a range you are 90% sure contains the answer.
Round 1
Did your range contain it?
- 1 Earth to Moon
- ≈ 384,000 km
- 2 Languages
- ≈ 7,100
- 3 Blood
- ≈ 5 L
- 4 Seconds in a day
- 86,400
- 5 Film frames
- 172,800
- 6 Heartbeats
- ≈ 2.7–2.8 billion
- 7 Drops per litre
- ≈ 20,000
- 8 China
- ≈ 1.41 billion
- 9 North Carolina
- ≈ 11 million
- 10 Cary
- ≈ 180,000
Round 1 · six students × 10 questions
What 90% should look like
Expected hits: 60 × 90% = 54
Actual hits: __Usually 22 to 30. "Your 90% is worth about 40%."
Why did the range miss?
"I did not know, so I guessed." "A wide range looks like I have no idea."
Not knowing is exactly when you should widen the range. A narrow range is not skill. Containing the answer is.
Method 1
Bounds first
- Write a number that is definitely too small.
- Write a number that is definitely too big.
- Then move each one inward only a little.
Most people do the opposite: think of an answer, then add a bit on each side.
Method 2
Break it into pieces
How many babies are born in the world each day? World population 8 billion × births per 1,000 per year 15 – 20 = per year 120 – 160 million ÷ 365 ≈ 330,000 – 440,000 per day True value: about 360,000 – 370,000
I had no idea of the answer. I knew rough population and birth rate; the range built itself, and it contained the truth.
Round 2 · in pairs · 5 questions
Now use the methods
- Every question: bounds first.
- At least two questions: write out the breakdown.
- 90 seconds per question.
1 / 5 · 90 s
Number of bubble-tea shops in Cary
–
Write a range you are 90% sure contains the answer.
2 / 5 · 90 s
Number of Starbucks stores in the United States
–
Write a range you are 90% sure contains the answer.
3 / 5 · 90 s
Breaths one person takes in a year
–
Write a range you are 90% sure contains the answer.
4 / 5 · 90 s
How long sunlight takes to reach the Earth
–
Write a range you are 90% sure contains the answer.
5 / 5 · 90 s
Share of the world population living in cities
–
Write a range you are 90% sure contains the answer.
Round 2
Answers, and one way to get there
- 1 Bubble-tea shops
- ≈ 9 – 11population ÷ people one shop can serve
- 2 US Starbucks
- ≈ 17,000340 million ÷ about 20,000 people per store
- 3 Breaths per year
- ≈ 8.4 million12–20 per minute × minutes in a year
- 4 Sunlight
- ≈ 8 min 20 s150 million km ÷ 300,000 km/s
- 5 Urban share
- ≈ 57%bounds first
Twenty minutes later
Round 1: __ / 60 ≈ 40% Round 2: __ / 15 ≈ 70 – 85%
You did not get smarter. You learned to say "I am not sure" correctly. That is the whole lesson.
The rule
Read it together
This week: add a confidence to every answer you give anyone, including "Did you hand in the homework? 90%." We check next Saturday.
Every answer is followed by "and here is how sure I am."
Every week from now on
The calibration scoreboard
| Confidence | Correct | Wrong |
|---|---|---|
| 50% | +1 | 0 |
| 70% | +2 | −2 |
| 90% | +3 | −6 |
Saying 90% is a bet: lose it and you pay six. Honest confidence scores the most; bragging and false modesty both lose.
1 / 5
The largest organ of the human body
- ALiver
- BSkin
- CLungs
How sure are you?50%70%90%
2 / 5
The speed of light, in kilometres per second
- A30,000
- B300,000
- C3,000,000
How sure are you?50%70%90%
3 / 5
Share of adults worldwide who can read
- A50%
- B70%
- C85%
How sure are you?50%70%90%
4 / 5
Number of giant pandas living in the wild
- A200
- B1,800
- C20,000
How sure are you?50%70%90%
5 / 5
World population in the year 1800
- A100 million
- B1 billion
- C3 billion
How sure are you?50%70%90%
Scoreboard
Score yourself
- 1 Largest organ
- B · skin
- 2 Speed of light
- B · 300,000 km/s
- 3 Adult literacy
- C · about 85%
- 4 Wild pandas
- B · about 1,800
- 5 World in 1800
- B · about 1 billion
Your notebook
First entry
For example: "I was 90% sure extreme poverty had doubled. It halved."
The thicker this notebook gets, the better. Next week you will find out that you only look for evidence that agrees with you. Don’t believe it? See you next week.
Week 1 — the moment I was fooled: ______