The Probability Scale
Understand probability as a measure on a scale from 0 (impossible) to 1 (certain); use the language of probability including likely, unlikely, certain, and impossible
What a learner can do afterwards
- Place everyday events on a 0-to-1 probability scale with justification
- Explain why a fair six-sided die gives each number a probability of 1/6
- Distinguish between equally likely outcomes (fair coin) and unequally likely outcomes (biased spinner)
1 · Read
Every event has a chance of happening. We measure that chance with a number called probability. Probability always sits between 0 and 1.
Words help you talk about where an event sits. At 0 we say impossible. Near 0, we say unlikely. Right in the middle, we say even chance. Near 1, we say likely. At 1 we say certain. An event and its opposite share the whole scale, so their two numbers add up to 1. If snow sits at 0.05, no snow sits at 0.95.
Compare three events. The sun rising tomorrow is certain, so its probability is 1. Flipping a coin and getting heads is an even chance, so its probability is 1/2. A fair die has 6 equally likely faces and only one shows a 6, so rolling a 6 has probability 1/6, which is about 0.17. A probability also predicts a count: multiply it by the number of tries. In 60 rolls of a fair die, expect about 60 × 1/6 = 10 sixes.
Fair means every outcome has the same probability, like heads and tails on a coin, or the six faces of a normal die. A biased spinner is not fair: one color covers more space than the others, so it is more likely to be landed on, and the outcomes are not equally likely.
Probability is a number from 0 (impossible) to 1 (certain), and words like unlikely, even chance, and likely tell you where an event sits on that scale.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
17 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.