Suppose a population contains 1,000,000 people, 5000 of whom take an illicit Dru
ID: 2927987 • Letter: S
Question
Suppose a population contains 1,000,000 people, 5000 of whom take an illicit Drug A. There is a blood test to check whether someone takes drug A. If someone takes the drug, it is positive (i.e. correct) 95% of the time. If someone does not take the drug, it is negative (i.e. correct) also 95% of the time. The table below shows the number of people who have the disease and test positive, etc.
B) Given that someone tests positive, what is the probability that the result is correct, i.e. that they do take Drug A?
C) Given that someone tests negative, what is the probability of the result is correct?
Positive Negative Total TestTest 47502505000 995,000 54,500945,5001,000,000 Has disease Does not have disease 49,750 945.250 TotalExplanation / Answer
a) P(Test is Correct) = 54500/1000000 = 0.0545
b) P(Correct/ Test Positive)
=4750/54500
= 0.087156
c) P( Correct/Negative)
= 250/945500
= 0.000264
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