A tree diagram is used in probability to show the possible combined outcomes of two, or more, experiments. Tree diagrams lend themselves well when only two outcomes from each experiment are of interest; for example, when rolling a dice, we'd normally consider six outcomes, but we may only be interested in whether we get a 6, so there would only be two events of interest - "6" and "not 6".
The first tree in a tree diagram would represent the outcomes (events) of the first experiment - we would write the probabilities of each along its branch. The second tree would follow on from each of the branches from the first tree, and itself would have two (or more) branches.
A key feature of tree diagrams is that they show how one event can follow on from another - this is particularly useful when the probabilities of the second experiment change (conditional probability) due to the outcome of the first experiment.
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