how sure are you?

Psychologists test overconfidence with a simple trick: ask people for a range, not an answer. If you say you are 90 percent sure that a number lies between your low and your high guess, you should be right about nine times out of ten. Almost nobody is. You do not need to be a trivia expert for this to work. You need to judge how wide your ignorance is.

For each question, give a low and a high guess so that you are 90% sure the true answer lies between them. Make the range as narrow as you honestly can, but wide enough that you would be surprised to miss. You should miss about 1 of 10.

1. In what year was the Eiffel Tower completed? (year)

   low high

2. What is the official height of Mount Everest above sea level? (metres)

   low high

3. How many bones are there in an adult human body? (bones)

   low high

4. What is the average distance from the Earth to the Moon? (km)

   low high

5. In what year was Magna Carta sealed by King John? (year)

   low high

6. How many member states does the United Nations have? (countries)

   low high

7. How fast does sound travel through dry air at 20 degrees Celsius? (metres per second)

   low high

8. How long is the Panama Canal from deep water to deep water? (km)

   low high

9. Around what year was the Gutenberg Bible printed? (year)

   low high

10. What was the original height of the Great Pyramid of Giza? (metres)

    low high