It's in the article. The 93% in that study includes a 59% true positive (of 93%, presumably) success rate. We can't know how many true or false negatives there

It's in the article. The 93% in that study includes a 59% true positive (of 93%, presumably) success rate. We can't know how many true or false negatives there are because those searches aren't actually carried out, so out of every hundred cars stopped for a dogsniff, 55% (59% of 93%) are found to be carrying drugs. More worryingly, that also means that out of every hundred cars stopped, 38 are wastefully searched by police (after getting probable cause from the positive dog alert) without finding drugs. From the article, a search takes about 45 minutes and carries a bunch of other risks for occupants of suspected cars (e.g. civil forfeiture, overenthusiastically trigger-happy LEOs), so this is pretty bad situation, not just in wasted time and cost, for both the car occupants and the police force. One slightly positive thing I'll take from this: Over 50% of cars that are dogged have drugs, and I sincerely doubt that over 50% of cars (on average) have drugs, so in this particular study it appears that police have a better than random chance of guessing which cars have drugs. It's not a very positive thing though, because civilized countries that operate under the rule of law cannot permit law enforcement to operate on guesswork. This is underlined by the fact that the article mentions that the false positive rate rose to something like 80% for latino drivers.