See Section IIA of: http://scholarship.law.duke.edu/cgi/viewcontent.cgi?article=.... It explains that probable cause is generally considered a lower standard than preponderance of the evidence (which is a 51% standard).
>We do not know exactly what the phrase “probable cause” means, in strict numerical terms. We do, however, know what it does not mean: “probably.” That is, probable cause does not—in the context of Fourth Amendment law—mean that the police must have evidence sufficient to conclude that a suspect is probably guilty or that she probably has evidence of a crime hidden inside her home.
I didn't read the whole paper, but the gist of it seems to being exploring the difference between 'concrete' and 'statistical' and evidence of effects (with a non-conclusive, but general indication that the supreme court leans towards 'concrete' over 'statistical')
Basically if you have a 3 people in a car that contains drugs and thus know 1 of them is guilty, you have probable cause to arrest all of them. (Concrete)
However, if you know that 1/3 of the black people in a certain neighborhood posses drugs, that does not mean you have probably cause to arrest all of them. (Statistical)
Thus my argument is that the low accuracy rate of the dog vs. the baseline of the searched people (which in this case is only known because of the high alert rate) should not allow it to constitute 'concrete' evidence of probable cause.
Additionally, the overall success rate of the dog/officer combination vs. the general population baseline constitutes 'statistical' evidence which is also not sufficient for probable cause.
Generally, my argument would be that allowing the dogs to serve as a final determination of probable cause without requiring high accuracy rates leads to unnecessary harm to the general populace in the form of the unwarranted searched that could be prevented by encouraging effective training and usage.