16. Given that a subject has tested negative for drug 1, what is the probability that the subject is not a drug 1 user? In other words, what proportion of those who tested negative for drug 1 were not actual drug 1 users? (Enter a number between 0 and 1). This probability is called negative predictive value (NPV) of the test. 17. Given that a subject has tested positive for drug 1, what is the probability that the subject is a drug 1 user? In other words, what proportion of those who tested positive for drug I were actual drug 1 users? (Enter a number between 0 and 1). This probability is called positive predictive value (PPW of the test