• Probability involves the study of the laws of chance and is a measure of the likelihood of an event happening.
• Rather than use words to describe the chance of an event happening, we can give probability as a number, usually written as a fraction or decimal, between 0 and 1.
• If it is impossible for an event to happen then the probability is 0.
• If an event is certain to happen then the probability is 1.
• All other probabilities are greater than 0 but less than 1.
• Sometimes probabilities are written as percentages, between 0 and 100%.
• To compare probabilities you must compare the relative sizes of the fractions, decimals or percentages.

• Consider a six sided die, on one throw of the die you can have 1 of 6 possible outcomes i.e. 1,2,3,4,5,6.
• If you want to find the event “The number on the die is an even number” then you would be interested in the numbers 2,4,6 . These numbers are said to be favourable outcomes.

• If, in an experiment each outcome is as likely to occur as any other outcome then you have what are called equally likely outcomes.
• It is possible to calculate the probability of an event happening using the following formula:

Probability of an event happening = (Number of favourable outcomes) / (Total number of outcomes)

When it comes to IGCSE/GCSE Maths, do you remember how to calculate the probability of the two main types of events?

•  Mutually exclusive events are events which cannot happen at the same time.

• If you roll a die the outcome will be a score of 1,2,3,4,5,6 but only one of these can occur, for example, the score 2 and 3 cannot occur at the same time.

• For any two mutually exclusive events A and B:

P(A or B) = P(A)+P(B)

The probability of an event not happening can be expressed thus: P(event does not happen) = 1- P(event happens)

• Independent events are events, which do not affect each other e.g. tossing a coin and throwing a die.

To find the probability that two independent events will happen we multiply their respective probabilities:

P(A and B) = P(A) * P(B)

• Listing outcomes can be a very hazardous job.
• A tree diagram is another way of listing outcomes and helps to simplify the calculation of probabilities when combined events are concerned.
• Each ‘branch’ of the tree indicates the outcome at each stage.
• Each route along the tree leads to a combined event.
• The tree is usually created across the page but can be drawn down the page.
• Consider a coin being tossed three times, the diagram below represents the possible outcomes, where H denotes a head and T a tail.

That's the end of the topic!

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Drafted by Bonnie (Mathematics)