The 0-to-1 probability scale

Mathematics · Probability · ages 10-11

Name ______________________ Date ____________

What is the probability of a certain event?
- 0
- 0.5
- 1
Write the fraction 1/2 as a decimal for the probability scale.
Answer: ______________
Jo says an event with probability 0.55 is likely because its probability is above 0.5. Is Jo right?
Circle one: True False
What is the probability of an impossible event?
- 0
- 0.5
- 1
Which of these probabilities sits closest to certain?
- 0.5
- 0.9
- 0.1
- 0
An event has probability 0.9. Which word fits it best?
- Unlikely
- Impossible
- Even chance
- Likely
An event is described as very likely. Which probability best matches that description?
- 0.40
- 0.95
- 0.05
- 0.60
Which probability best matches the phrase 'very unlikely'?
- 0.1
- 0.5
- 0.9
A probability is written as 75%. Write it as a decimal.
Answer: ______________
Ravi sees a weather forecast showing a 90% chance of rain and says that means rain is 'very unlikely'. What mistake did he make?
- He thought a percent cannot be placed on the 0 to 1 probability scale at all
- He mixed up which end of the scale means likely and which means unlikely
- He thought only a 100% chance of rain can ever be called likely

Answer key

For grown-ups. Fold this page away before handing over the rest.

1 · Certain events sit at the very end of the scale, 1.
0.5 · 1/2 as a decimal is 0.5.
True · 0.55 is above the middle value of 0.5, so it is on the likely side of the scale.
0 · Impossible events sit at the very start of the scale, 0.
0.9 · Certain is at 1. The gap from 0.9 to 1 is 0.1, smaller than the gap for any other choice.
Likely · 0.9 is above 0.5, on the likely side of the scale. Of the four words, only likely belongs to that side.
0.95 · Very likely means close to the certain end at 1. Of these values, 0.95 is closest to 1.
0.1 · 'Very unlikely' means close to the impossible end of the scale, near 0.
0.75 · 75% means 75 out of 100, which is the fraction 75/100. That is 75 hundredths, written 0.75.
He mixed up which end of the scale means likely and which means unlikely · Numbers close to 1 (or 100%) are near the certain end, so 90% means rain is very likely, not unlikely.