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For the first branch, refer to your answers above to obtain the following:
The probability that a randomly chosen athlete uses steroids is P(F) = 0.1.
Since the sum of the probabilities on the branches leaving a node is 1, the other branch must be labeled P(F') = 0.9, giving the following first stage:
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Starting at the "S"-node (given that an athlete uses steroids) the probability that the althlete tests positive is P(E|F) = 0.95, and the probability that he or she teste negative is 0.05. This gives the next stage:
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Starting at the "not S"-node (no steroid use) the probability of a (false) positive is P(E|F') = 0.15. Since the branches leaving a node must sum to 1, P(E'|F') = 0.85. This leads to the completed tree:
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