STAT 1100Q Chapter Notes - Chapter 6: Immunoassay, Antibody, Enzyme
22 May 2018
School
Department
Course
Probability Examples
Name: ____________________ Section: ________
I. Testing for HIV: 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 not always correct. Here
are approximate probabilities of positive and negative EIA outcomes when the blood
tested does and does not actually contain antibodies to HIV:
Test Result:
+
-
Antibodies Present
0.9985
0.0015
Antibodies absent
0.0060
0.9940
Suppose that 1% of a large population carries antibodies to HIV in their blood.
(a.) Draw a tree diagram for selecting person from the population and testing his or
her blood.
.009985 P( Person is HIV +ve AND test is +ve)
.000015
P( Person is HIV +ve AND test is -ve)
Person
.00594 P( Person is HIV -ve AND test is +ve)
.98406 P( Person in HIV -ve AND test is -ve)
(b.) What is the probability that the EIA test is positive for a randomly chosen person
from this population?
P( TEST=+VE) = P( Person is HIV +ve AND test is +ve) + P( Person is HIV -ve AND
test is +ve)
.009985+.00594= .015925
(c.) What is the probability that a person has the antibody, given that the EIA test is
positive?
.01
.99
.9985
.0015
.0060
.9940
find more resources at oneclass.com
find more resources at oneclass.com
2
STAT 1100Q Full Course Notes
Verified Note
2 documents
Document Summary
Name: ____________________ section: _______: testing for hiv: enzyme immunoassay (eia) tests are used to screen blood specimens for the presence of antibodies to hiv, the virus that causes aids. The test is quite accurate but not always correct. Here are approximate probabilities of positive and negative eia outcomes when the blood tested does and does not actually contain antibodies to hiv: Suppose that 1% of a large population carries antibodies to hiv in their blood. (a. ) Draw a tree diagram for selecting person from the population and testing his or her blood. 009985 p( person is hiv +ve and test is +ve) P( person is hiv +ve and test is -ve) . 00594 p( person is hiv -ve and test is +ve) . 98406 p( person in hiv -ve and test is -ve) P( test=+ve) = p( person is hiv +ve and test is +ve) + p( person is hiv -ve and test is +ve)
