The 0-to-1 Probability Scale
Understand probability as a measure expressed as a number between 0 (impossible) and 1 (certain); place events on the probability scale; express probabilities as fractions, decimals, and percentages
What a learner can do afterwards
- Place events on a number line from 0 (impossible) to 1 (certain), expressing positions as fractions or decimals
- Explain that a probability of 0.5 means 'even chance', and that values above 0.5 match the informal word 'likely'
- Convert between informal language ('very unlikely') and a numerical position on the 0-to-1 scale
1 · Read
Probability is a number that tells you how likely something is to happen. It always sits somewhere between 0 and 1. An event that can never happen has a probability of 0. An event that will always happen has a probability of 1.
A weather app shows a 5 in 10 chance of rain today. That's the same as 1/2, 0.5, or 50%. Because 0.5 sits right in the middle of the scale, rain and no rain are equally likely, which is an even chance.
The same chance can be written three ways: a fraction such as 6/10, a decimal such as 0.6, or a percent such as 60%. Percent means out of 100, so 40% is 40/100, which is 0.4, and 35% is 35/100, which is 35 hundredths, written 0.35. Words match places on the scale. Close to 0 is very unlikely. Below 0.5 is unlikely. Exactly 0.5, which is 1/2 or 50%, is an even chance. Above 0.5 is likely. Close to 1 is very likely.
Every probability lands somewhere between 0 (impossible) and 1 (certain), and that same spot can be written as a fraction, a decimal, or a percent.
One reading is adapted from the Khan Academy article 'Key ideas: What is probability?'; its terms are unknown. Linked Khan Academy videos: 'Comparing two probability values expressed in different forms' and 'Fraction, decimal, and percent from visual model'. The reading 'One spot, three names' and the eight practice questions were written for os-taxonomy.
2 · Watch
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Where it sits
Where this leads
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16 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.