The ELISA HIV Blood Test

Application example for probabilities

ELISA is a test for AIDS used to screen donated blood for hiv.

(See http://www.stat.ucla.edu/cases/elisa/ )

• Here's a simplified scenario for 1,010,000 people donating blood and the results on their blood:

Randomly pick one person from these 1,001,000. Fill in the blanks below:

1. The chance the person has AIDS is____________________ 2. The chance the person tests positive given the person has AIDS is

_________3. The chance the person tests positive given the person does not have

AIDS is_____ 4. Are "having AIDS" and "testing Positive" independent? _______ _______________________________________________

5. The chance the person has AIDS given the person does not test positive is__________

6. The chance the person has AIDS given the person tests positive is____________

7. Testing positive means:5. The person almost certainly has AIDS 6. The person probably had AIDS 7. The person has a fifty-fifty chance of having AIDS 8. The person probably doesn't have AIDS, but may 9. The person definitely does not have AIDS.

• Answers: