The Centers for Disease Control and Prevention receives information about the causes for HIV/AIDS infection in the United States. Here are the causes of infection and their respective probability among American women ages 20 to 24 years old.

CauseProbability

Injection drug use?

Heterosexual sex0.664

Perinatal infection0.125

Other causes0.095

Reference: Ref 9-2

This is a legitimate probability model. Therefore, the probability that an American woman age 20 to 24 was infected via injection drug use is

0.116.0.186.0.206.0.250.