As prt of a training program, staff at a testing lab applied a screening test to 500 blood samples known to contain HIV. The test was postive for 489 blood samples and negative for 11 samples. as of the same training program, the staff also applied the screening test to 500 other blood samples known to not to contain HIV. The test was positive for 37 blood samples and negative for 463. a. When a new blood sample arrives at the lab, it will be tested to determine whether HIV is present. Using the data from the training program estimate the probability that the screening test would be positive when it is applied to a blood sample that does not contain HIV.
b. Among the blood samples examined in the training program that provided positive screen results for HIV what proportion actually contained HIV?