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not use steroids. a true positive means the athlete tested positive and used steroids. what is the probability the athlete used steroids, given that 1% of athletes use steroids, the test's false positive rate is 0.5%, and the test's true positive rate is 99%?

Sagot :

The probability that an athlete used steroids, given that they tested positive, is approximately 0.5025%.

In order to calculate the probability that an athlete used steroids given the information provided, we need to know the probability that they tested positive. To do this, we can use the formula for conditional probability:

P(A|B) = P(A and B) / P(B)

In this case, we are trying to find the probability that an athlete used steroids (A) given that they tested positive (B). We know that the probability of A is 1% and the probability of B is 0.5%, so we can calculate P(A and B) by multiplying these two probabilities together:

P(A and B) = 1% * 0.5% = 0.005%

Next, we need to calculate P(B), which is the probability that the athlete tested positive. Since we know the false positive rate and the true positive rate of the test, we can use these to calculate P(B) using the formula:

P(B) = P(A and B) + P(not A and B)

P(B) = P(A|B) * P(B) + P(not A|B) * P(B)

P(B) = 0.99 * 0.005% + 0.005 * (1 - 0.005%)

P(B) = 0.00495% + 0.005 * 0.999

P(B) = 0.0099495%

Now that we have P(A and B) and P(B), we can plug these values into the formula for conditional probability to calculate the probability that an athlete used steroids, given that they tested positive:

P(A|B) = P(A and B) / P(B)

P(A|B) = 0.005% / 0.0099495%

P(A|B) = 0.5025%

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