Class 6 HIV Testing


  • 1. In one of Marilyn vos Savant's columns in Parade Magazine the following question was asked.

    Suppose we assume that 5% of the people are drug-users. A test is 95% accurate, which we'll say means that if a person is a user, the result is positive 95% of the time; and if she or he isn't, it's negative 95% of the time. A randomly chosen person tests positive. Is the individual highly likely to be a drug-user?
    Marilyn's answer was:
    How can Marilyn's answer be correct?

  • 2. An article in the New York Times some time ago reported that college students are beginning to routinely ask to be tested for the AIDS virus.

    The standard test for the HIV virus is the Elisa test that tests for the presence of HIV antibodies. It is estimated that this test has a 99.8% sensitivity and a 99.8% specificity. 99.8% specificity means that, in a large scale screening test, for every1000 people tested who do not have the virus we can expect 998 people to have a negative test and 2 to have a false positive test. 99.8% sensitivity means that for every 1000 people tested who have the virus we can expect 998 to test positive and 2 to have a false negative test.