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Conditional probability is the probability of an event given another has occurred. This topic calculates it from tree diagrams, two-way tables and Venn diagrams. It's examined right across GCSE and iGCSE Maths. Work through it free on eClassroom, with clear worked examples and instantly-marked questions that build your confidence for the exam.
How you do it
You are told one thing has already happened, so the sample space shrinks to just those cases. Divide the probability of both events by the probability of the one you are given. On a tree, this means using the second set of branches directly.
Before you start
Bring tree diagrams and the AND rule with you. It underpins harder combined-event questions. It appears at Higher tier.
What you learn
Conditional probability is the chance of one event given that another has already happened — you only look within the group you’re told about. Conditional probability is the chance of B given that A has happened. In a without-replacement tree, each second-stage branch is a conditional probability — the chance of the second event given the first. The course works through ‘Given that’ restricts the total, conditional probability in a tree and using ‘given’ in context. Every section has instant-feedback practice questions matched to Edexcel iGCSE 4MA1 and the other major GCSE boards, and the course finishes with full exam-style questions you mark yourself.
Related Topics
Conditional Probability is part of the Probability strand of our free GCSE maths courses, studied in Higher tier. The course is built from 3 concept sections, 2 worked examples and 3 check points before the exam-style set. Related topics students usually take next include Expected Frequency, Mutually Exclusive Events, Relative Frequency, Sample Space Diagrams and Tree Diagrams. The Probability revision engine gives you as many extra questions as you want, tracked on your skill tree.
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