Question 1

Paper 1, no calculatorMedium7 marks

A probability tree models the results of a medical study of a risk factor RR and a disease DD A person is selected at random from the population. The probabilities on the second set of branches are conditional on the outcome at the first set of branches.

Selected personRnot RDnot DDnot D154518781403940
(a)

Find the probability that a randomly selected person has both RR and DD

[1]
(b)

Hence determine the probability that a randomly selected person has DD

[2]
(c)

Hence determine the probability that a person has RR given that they have DD Give your answer exactly, and state whether more than half of the people with DD have RR

[4]

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