Looking under the lamppost

Stuff is reporting on new drug tests being pushed by NZDDA

Hardy said hair testing was more accurate and effective method of detecting drug use, and it gave a history of drug or alcohol use over the previous 90 days….With urine tests more drugs were undetectable if urinalysis was carried out more than three days after use.

Since the advertised purpose of employee drug testing is to catch people who are impaired on the job, expanding the history from 3 days to 90 days surely makes the test less accurate, not more accurate.  It’s more accurate only if you don’t distinguish between on-the-job and off-the-job drug and alcohol use — like the drunk looking for his keys under the lamppost because he could see better there.

One of the key contributions of statistics to evidence-based medicine has been in forcing medical researchers to measure what they really want to affect, not what is convenient and plausibly related to it.  Drug use in the past 90 days is not the same as on-the-job impairment, and it’s probably a pretty lousy surrogate.

The Assistant Privacy Commissioner is quoted in the article as saying

“Employers should only use it where there is a genuine business need. For example, drug testing has been allowed where there are safety issues with operating machinery.

An interesting approach used by some US companies is drug testing after accidents.  A study from Princeton (PDF) found that this did reduce accidents by about 10%, though some of the reduction may have been due to under-reporting of accidents — an important tradeoff to consider.

There isn’t going to be a quick technological fix, however, and we do need some sort of regulation. As Stanford’s Keith Humphreys puts it

…we use public policy to pick the particular sort of drug problem society will have. For example, different policy environments can make it a human rights problem, an addiction problem, a crime problem, an AIDS problem, a public disorder problem etc., but no policy will produce a true ending of all of society’s problems with drugs. There are some policies that ameliorate multiple aspects of the problem, but in most cases we are faced with hard choices about what sort of problem we will have rather than a problem-free alternative.