“In a roadside study, one in three reckless drivers who were tested for drugs tested positive for marijuana. It’s more harmful than we all thought.”
Gnrgh! Meaningless! How many other drivers tested positive for marijuana? Was the ratio of reckless to non-reckless drivers different for those testing positive than those testing negative? Were all the reckless drivers stopped tested? The language quoted implies that they weren’t, so what factors determine who were tested and who weren’t?
You know what’s harmful? Public awareness campaigns that encourage people to think sloppily about statistics, that’s what. These are the basic tools we need in order to make sense of the flood of information all around us, and ads like this rely on our failure to understand elementary statistics and survey methods. Pisses me off, if that wasn’t obvious.