Definition
Probability scale
A probability scale is a line from 0 to 1 that shows how likely an event is.
View exact chart data
| Probability | Meaning |
|---|---|
| 0 | Impossible |
| 0.25 | Unlikely |
| 0.5 | Even chance |
| 0.75 | Likely |
| 1 | Certain |
What it means
Move from left to right and the chance grows. The ends are fixed: impossible at 0 and certain at 1. Halfway, at 1/2 or 0.5, means the event is just as likely to happen as not happen. Words such as unlikely and likely tell an order, but they do not have one fixed number unless the scale says where to put them.
Remember it
Think of a chance slider: left is impossible, the middle is fifty-fifty, and right is certain.
Example
For a fair coin, the probability of heads is 1/2, so heads sits at the middle of the scale. Rolling a 7 on a standard six-sided die is impossible, so it sits at 0. Rolling a number from 1 to 6 is certain, so it sits at 1.
Watch for
A probability scale measures likelihood, not how large, fast, or important something is. A valid probability cannot be less than 0 or greater than 1.
Common mix-up
The word “likely” does not always mean exactly 3/4. It names a region toward the certain end unless a particular scale defines a precise position.
Exact meaning
A probability scale orders an event’s probability from 0 to 1: 0 means impossible, 1/2 means an even chance, and 1 means certain. Fractions, decimals, and percentages can name the same position on the scale.