ATCs are required to undergo periodic random drug testing. A simple, low-cost urine test is used for initial screening. It has been reported that this particular test has a sensitivity and specificity of 0.96 and 0.93. This means that if there is drug use, the test will detect it 96% of the time. If there is no drug use, the test will be negative 93% of the time.
Based on historical results, the FAA reports that the probability of drug use at a given time is approximately 0.007 (this is called the prevalence of drug use).
Draw a probability tree for the situation. (Use this order for the tree: Drug Use(yes/no) -> Test Result(+/-) -> Joint Probabilites)
A positive test result puts the air traffic controller’s job in jeopardy. What is the probability of a positive test result?
Find the probability an air traffic control truly used drugs, given that the test is positive.