TBO Fail: Selection Bias and Voluntary Drug Testing

The Florida state legislature, at Governor Rick Scott's urging, passed strict legislation earlier this year requiring welfare recipients to take mandatory drug tests--and pass--as a requirement for receiving their state benefits.  Tampa Bay Online ran a story today reporting that only 2% of welfare recipients have failed their drug tests thus far.  That's a surprisingly low number.  However, the author of the article draws a faulty conclusion from this figure:

The initiative may save the state a few dollars anyway, bearing out one of Gov. Rick Scott's arguments for implementing it. But the low test fail-rate undercuts another of his arguments: that people on welfare are more likely to use drugs.

As much as I would love this conclusion to be true, it suffers from a flaw known to most social scientists--selection bias.  Specifically, individuals most likely to fail a drug test are not going to take the test.  The drug test is voluntary, after all.  Thus, the TBO's attempt to draw a larger conclusion from this figure is extremely dubious.