For occasional marijuana users who have always managed to pass pre-employment drug tests by the sophisticated strategy of not smoking pot for a few weeks before handing over their urine, the prospect of widespread hair analysis is worrisome. Unlike urine testing, which stops finding traces of marijuana, even in heavy users, after a month or so, hair testing potentially can reveal a drug use history going back months or even years, depending on how long the hair has been growing. But NORML reports that a study to be published by Forensic Science International offers new hope to weekend pot smokers, finding that hair testing fails to detect consumption of one to five joints per week almost half the time. For daily smokers, the detection rate was 85 percent. Even the 1 in 2 chance of being outed as a pot smoker could be daunting to job applicants, of course, but the high error rate may discourage employers from using hair testing (which is substantially more expensive) to begin with. A few years ago in Reason, I tried to figure out why employers do drug testing at all.