title: Probability Scale

 

Mathematicians use a probability scale. This puts a numerical value against the likelihood of a particular outcome happening.

Move the mouse over the scale to see what the numbers show or click for a text description.

 

image: probability scale showing a range from 0 (not probable) 1 (certain) 0.5 1 0

 

Probabilities and outcomes

When an event has more than one outcome, what do the probabilities of each outcome add up to?

Looking at the roll of a dice:

The probability of throwing a 1 is 1 in 6. This can be written as 1/6 or 0.1666.

The probability of throwing a 2 is 1 in 6. This can be written as 1/6 or 0.1666.

Think about the probabilities of rolling a 3, 4, 5, or 6.

So if you look at all the possible outcomes, the total of all their probabilities must add up to 1. This could be thought of by saying that it is absolutely definite that when you roll a dice you wil get a 1 or 2 or 3 or 4 or 5 or 6.

Knowing one probability allows another to be calculated

Remember that the total of probabilites for a particular event add up to 1. So what are these probabilities?

 

 

If the probability of it snowing in London on Christmas day is 0.2, what is the probability that it will NOT snow?

0.2
0.3
0.8
0.6

The probability of rolling an even number on a six-sided dice is 3/6. What is the probability of rolling an odd number?

3/6
1/6
4/6
1/3

Percentage probabilities

Just to make things really tricky, probabilities are sometimes also quoted in percent. To do this just use a probability scale that runs from 0% (never happen) to 100% (definitely happen). Remember that the total of all probabilities must now add up to 100%.

image: probability percentage scale showing 0% (not probable) to 100% (probable)