Events and outcomes
An event is something that might happen, like "spinning red" or "rolling an even number". Each possible result is an outcome. When you toss a coin, the outcomes are heads and tails.
The probability scale
We describe how likely an event is with words:
| Word | Meaning |
|---|---|
| Impossible | Never happens. |
| Unlikely | Less than an even chance. |
| Equally likely / even chance | Just as likely to happen as not. |
| Likely | More than an even chance. |
| Certain | Always happens. |
Spinners show chance clearly
| Spinner | Event: land on red |
|---|---|
| All red | Certain |
| 3 of 4 equal parts red | Likely |
| 2 of 4 parts red | Equally likely to land on red or not |
| 1 of 4 parts red | Unlikely |
| No red | Impossible |
The bigger the red area, the more likely red is.
Comparing events
Sometimes we compare two events: which is more likely?
A bag has 6 blue marbles, 3 yellow and 1 green.
- Blue is the most likely (6 out of 10).
- Green is the least likely (1 out of 10).
- Picking purple is impossible: there are no purple marbles.
- Picking a marble is certain.
Equally likely
Outcomes are equally likely when each has the same chance. On a fair coin, heads and tails are equally likely. On a fair die, each number from 1 to 6 is equally likely.
Testing with experiments
You can test chance by trying it many times. Spin a spinner 20 times and tally where it lands. The most likely colour should come up most often, but results can surprise you, especially with only a few tries.
Worked examples
Example 1: You roll a die. Is rolling a number less than 7 certain, likely, unlikely or impossible?
All numbers 1 to 6 are less than 7: certain.
Example 2: A bag has 9 red cubes and 1 blue cube. Describe picking blue.
Unlikely.
Example 3: Are heads and tails equally likely on a fair coin?
Yes.
Common mistakes
- Thinking a result is "due". If you flip heads 4 times, tails is still only equally likely on the next flip.
- Comparing only colours, not amounts. Count how many of each there are.
- Calling unlikely events impossible.
Try it yourself
- A spinner is half red and half blue. Is red likely, unlikely or equally likely?
- You pick from a bag of only green balls. Is getting green certain or likely?
- A bag has 2 red and 8 white balls. Which colour is more likely?
- Rolling a 1 on a die: likely or unlikely?
Answers: 1) equally likely 2) certain 3) white 4) unlikely
Quick recap
- An event can be impossible, unlikely, equally likely, likely or certain.
- More of something means a bigger chance of picking it.
- Past results don’t change the chance of the next one.