What is it
At Higher tier, probability trees extend to conditional probability — where the probability of a second event depends on the outcome of the first, such as selecting items without replacement.
Why it matters
Conditional probability with 'without replacement' scenarios is one of the most reliably challenging topics at GCSE Higher, precisely because it requires updating your thinking based on what's already happened — a genuinely different skill from independent probability.
Exam tip
For 'without replacement' problems, always recalculate both the numerator and the denominator for the second branch of the tree — forgetting that the total also changes (not just the count of the item removed) is the single most common error.
Related topics
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