MEPX
Chapter 1 of 11All chapters
  1. 1What probability means
  2. 2Counting outcomes
  3. 3Independent events
  4. 4Dependent events
  5. 5Complements and at least one
  6. 6Adding probabilities
  7. 7Permutations and combinations
  8. 8Conditional probability
  9. 9Expected value
  10. 10Distributions in outline
  11. 11Fallacies and traps

Chapter 1 of 11

What probability means

A number between zero and one.

The scale

Probability runs from 0, which is impossible, to 1, which is certain. A half means the event happens in half of all possible worlds, not that it will alternate.

  • Percentages, fractions and decimals are the same thing written differently.
  • Odds are a different scale: 3 to 1 against means a probability of one quarter.

Where the numbers come from

Sometimes from symmetry, as with a fair die. Sometimes from data, as with weather records. Sometimes from judgement, which is honest as long as you say so.