Question 9
6 pts
In the US, testing initially was very selective. In other words, as of early April 2020, only patients (i) with
symptoms (ii) who contacted the health care system were being tested. For the most part, tests were not
obtainable on demand, and there was very limited testing of asymptomatic people, even if they had been in
contact with someone who had tested positive.
When testing is selective, then for interpreting results of testing, what matters is not the fraction of the
entire population who are infected, but rather the fraction of the tested population who are infected.
If the prevalence of infection in the tested population is 0.8 (in other words, if 80% of people tested have
the infection), what is the probability that a person who tests positive actually is infected? 3 decimals
0.996
