suppose that you walking down the street and notice that the department of public health is giving a free medical test a certain disease. the test is 90 percent reliable in the following sense : if a person has the disease, there is a probability of 0.9 that the test will give a positive response ; whereas, if a person dose not have the disease, there is a probability of only 0.1 that the test will give a positive response. this same test was considered in example 2.3.1. we shall let X stand for the result of the test, here