In: Statistics and Probability
Diagnostic tests of medical conditions can have several results. 1) The patient has the condition and the test is positive (+) 2) The patient has the condition and the test is negative (-) – Known as “false negative” 3) The patient doesn’t have the condition and the test is negative (-) 4) The patient doesn’t have the condition and the test is positive (+) – Known as “false positive” Consider the following: Enzyme immunoassay (EIA) tests are used to screen blood specimens for the presence of antibodies to HIV, the virus that causes AIDS. Antibodies indicate the presence of the virus. The test is quite accurate but is not always correct. Suppose that 1% of a large population carries antibodies to HIV in their blood. Of those that carry the HIV antibodies in their blood, 99.85% will have a positive test result and 0.15% will have a false-negative test result. Of those that do not carry the HIV antibodies in their blood, 99.4% will have a negative test result and 0.60% will have a false-positive test result. Draw a tree diagram for selecting a person from this population and testing his or her blood. Take a look in the example on page 398 of the class text book for an example of a tree diagram. b) Construct a probability table that shows the probabilities for individuals in this population with respect to the presence of antibodies and test results. Take a look in the example on page 394 of the class text book for an example of a probability table. c) What is the probability the EIA is positive for a randomly chosen person from this population? d) In words, define the sensitivity of a test like this. Define the sensitivity in the context of this test using conditional probability notation. Calculate the sensitivity of this test? (you may need to look up what this term means for this context) Take a look at the last equation/calculation in the right hand column of the example on page 398 of the class textbook for an example of the conditional probability notation. e) In words, define the specificity of a test like this. Define specificity in the context of this test using conditional probability notation. Calculate the specificity of this test? (you may need to look up what this term means for this context) Take a look at the last equation/calculation in the right hand column of the example on page 398 of the class textbook for an example of the conditional probability notation. f) In words, define the positive predictive value of a test like this. Define positive predictive value in the contest of this test using conditional probability notation. Calculate the positive predictive value of this test? (you may need to look up what this term means) Take a look at the last equation/calculation in the right hand column of the example on page 398 of the class textbook for an example of the conditional probability notation. g) In words, define the negative predictive value of a test like this. Define negative predictive value in the context of this test using conditional probability notation. Calculate the negative predictive value of this test? (you may need to look up what this term means) Take a look at the last equation/calculation in the right hand column of the example on page 398 of the class textbook for an example of the conditional probability notation.
Let E1 be the event that a person carries antibodies to HIV in their blood
Let E2be the event that a person does not carry antibodies to HIV in their blood.
Let A be the event that the test is positive
Let B be the event that the test is negative.
Probability tree
b) P(E1)=0.01 P(E2)=0.99 P(A/E1)=0.9985 P(A/E2)=0.006 P(B/E1)=0.0015 P(B/E2)=0.994
c)
d) Sensitivity is in a population of people actually carries the antibodies, proportion of people test positive
Specificity is from the pop[ulation people actually not carries the antibodies, proportion of people test negative.
e) Positive predictive value is the proportion of actually diseased people among positive tested people.
f) Negative predictive value is the proportion of actually not diseased people among negative tested people.