Probability
The likelihood scale
Some things are certain, some impossible, and most sit somewhere in between. The likelihood scale gives you the words to say where.
The five words
The LIKELIHOOD SCALE runs from left to right: IMPOSSIBLE — it cannot happen. UNLIKELY — it probably will not happen. EVEN CHANCE — just as likely to happen as not. LIKELY — it probably will happen. CERTAIN — it definitely will happen. Every outcome can be placed somewhere on that line.
Impossible and certain
These are the two ends, and they are absolute. Rolling a 7 on an ordinary dice is IMPOSSIBLE — there is no 7 on it. Rolling a number from 1 to 6 is CERTAIN — every face is one of those. If something has even the smallest chance, it is not impossible. And if it could possibly fail, it is not certain.
Even chance
An EVEN CHANCE means the outcome is just as likely to happen as not to happen. Flipping a fair coin and getting heads is an even chance: there are two outcomes, and both are equally likely. Be careful — two possible outcomes does not automatically mean an even chance. Rain tomorrow either happens or does not, but that does not make it an even chance.
Placing outcomes on the scale
Draw the scale as a line and put an arrow where each outcome belongs. Rolling a 3 on a dice: there is 1 chance in 6, so UNLIKELY. A coin landing on tails: EVEN CHANCE. Rolling an even number on a dice: 3 of the 6 faces, so EVEN CHANCE. Rolling a number less than 6: 5 of the 6 faces, so LIKELY. Count how many of the possible outcomes give you what you want.
Worked examples
Rolling a 3 on a dice
- A dice has 6 faces, all equally likely.
- Only one of them is a 3.
- One chance out of six is small.
Answer: Unlikely
A coin landing on tails
- A coin has two outcomes: heads or tails.
- Both are equally likely.
Answer: Even chance
Rolling a 7 on an ordinary dice
- The faces are numbered 1 to 6.
- There is no 7 on the dice at all.
Answer: Impossible
Rolling a number from 1 to 6
- Every face on the dice shows a number from 1 to 6.
- So whatever you roll, it will be one of them.
Answer: Certain
Rolling an even number
- The even faces are 2, 4 and 6 — that is 3 of them.
- The odd faces are 1, 3 and 5 — also 3.
- The two are equally likely.
Answer: Even chance
Where you see this in real life
- A weather forecast saying rain is 'likely' rather than certain.
- It is impossible for the sun to rise in the west.
- Flipping a coin to start a match is fair because it is an even chance.
Key words
- Likelihood
- How likely something is to happen.
- Impossible
- Cannot happen at all.
- Unlikely
- Probably will not happen.
- Even chance
- Just as likely to happen as not.
- Likely
- Probably will happen.
- Certain
- Definitely will happen.
- Outcome
- One of the things that could happen.