Example: Suppose that Walmart has a mandatory drug test for all employees. The test attempts to identify users of illegal drugs, such as cocaine. Let's assume that the test is 95% accurate. This means that 95% of time a positive test result will accurately identify a drug user. Similarly, 95% of the time a negative test result will accurately identify a non-drug user. Let's assume that 4% of the employees are drug users. If the drug test is positive, what is the probability that the test is wrong, and the employee is not using drugs? We want to find a conditional probability, P(drug user no | positive test). To find this probability we will construct a two-way table for a hypothetical sample of 10,000 employees. (10,000 is a large enough number to ensure that all calculations result in whole numbers.) Drug User Yes Drug User No Totals Drug Test: Positive Drug Test: Negative Totals Which two cells in the table will we use to calculate P(drug user no | positive test)? Take notes as we complete the table together. P(drug user no | positive test) =