While we agree that illegal use of drugs is a serious problem, your articles about large-scale drug testing in the workplace indicate that such testing has potential for great harm. Questions of invasion of privacy and other injury to civil liberties are substantial.
We want to amplify your discussion by noting that mass screening, however accurate on its face, may carry unacceptably high risks to the reputation and jobs of large numbers of innocent people. The phenomenon of so-called false positives , mentioned in your articles, is well known to professionals in survey work. The issue of quantitative determination of the percentage of positive tests that are "false" demands a place in the current public debate.
Readers may be surprised at how serious the problem is. Simple arithmetic is enough to see this. Suppose that 10% of the population uses drugs for which a test is to be administered (in line with the 9%-12% figure for drugs in the workplace mentioned in your articles). Suppose also that the test is 95% accurate in reporting the status of either a drug user or a drug-free person (in line with an estimate of accuracy of 95%-97% given in your article as a good figure for a test with confirmation ). With an "on average" analysis, among 1,000 employees, 100 are drug users and 95 will be reported as such by the test. Of the remaining 900 employees, 45 (5%) will be reported as drug users. Thus, of the 140 employees with positive reports, nearly 1 out of 3 will be erroneously labeled as drug users.