A Level Maths: Conditional Probability Topic Summary and Resources
Video Lessons
Watch alongside the worksheet for the full lesson experience, then test your understanding with the lesson questions.
Revision Notes
Handwritten notes summarising the key ideas for each lesson. Ideal for quick review before a test.
Exam Questions
Past-paper-style questions organised by topic, with full mark schemes.
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Drawn from OCR and Edexcel past papers but designed to be useful for students of all UK exam boards — including AQA and OCR MEI — unless a sheet is explicitly board-specific.
Before You Start This Topic
It will help if you are confident with the following:
- Probability — foundational probability is essential
- Discrete Distributions (inc. Binomial Distribution) — useful context for conditional events
- GCSE Maths Probability — tree diagrams from GCSE
A Level Maths conditional probability is the Year 2 extension of Probability, focused on probabilities that depend on prior information. The notation $P(A|B)$ means 'the probability of $A$ given that $B$ has happened', and conditional probability is the framework for analysing dependence between events, two-way tables, and Bayes-style reasoning.
The key formula is $P(A|B) = \frac{P(A \cap B)}{P(B)}$, which you rearrange as the multiplication rule $P(A \cap B) = P(A|B) \cdot P(B)$. You use this on tree diagrams with branches representing conditional probabilities, and on two-way (contingency) tables where you read off counts and convert to conditional probabilities. You test for independence: events $A$ and $B$ are independent if $P(A|B) = P(A)$, equivalently $P(A \cap B) = P(A) \cdot P(B)$. You apply conditional probability to real contexts — medical screening (with sensitivity and specificity), faulty production lines, weather forecasting — where the headline probability changes once you know more about the situation. You also handle Venn diagrams where set sizes are given, computing conditional probabilities directly from the overlap structure.
Conditional probability is part of the Statistics strand of A Level Maths for AQA, Edexcel, OCR, and OCR MEI students.
Watch out for…
A few things to be careful with: $P(A|B)$ and $P(B|A)$ are usually DIFFERENT — confusing them is one of the most common errors in probability, especially in medical-testing contexts; the formula $P(A|B) = \frac{P(A \cap B)}{P(B)}$ requires $P(B) > 0$, so check for that; independence is a SPECIFIC mathematical condition — events that 'feel' unrelated may not actually be independent; and when reading tree diagrams, the probabilities along the branches AFTER the first level are conditional on the path taken, not unconditional.