Of the many things that make my Criminal Procedure students cynical about the Supreme Court, perhaps the most frustrating is that the Court has refused to quantify the probable cause standard. The Supreme Court’s grant of certiorari last week in Florida v. Harris gives the Court the perfect opportunity to at least place probable cause within some numerical band.
Harris is a particularly good vehicle for making the probable cause standard less fuzzy. In Harris, the Florida Supreme Court confronted the issue of when a dog’s positive alert gives the police probable cause to search a vehicle. Unlike in most assessments of probable cause, which involve informants or suspicious seeming individuals, police have data that quantifies the accuracy of drug sniffing dogs. A dog’s field history includes its rate of false positives, when a dog alerts to the smell of drugs that are not actually present in the vehicle. The Florida Supreme Court held that a dog’s field history must be introduced as part of the probable cause inquiry. If the lower court’s opinion is upheld, the Supreme Court should tell us what sort of false positive rate is too unreliable to permit a full search of a car.
Courts consistently and expressly eschew technical conceptions of probable cause in order to provide police officers with flexibility to exercise their judgment in unfolding situations. In addition, courts focus on whether an officer has a reasonable belief that a suspect has committed or is committing a crime. This metric allows for probable cause to be found in situations where one reasonable officer might assess an 80% likelihood that a suspect is driving drunk, for example, even if another reasonable officer might think there is only a 40% likelihood. We might be tempted to assume the courts require that a reasonable officer be able to believe a crime has been committed by greater than a 50% likelihood, but this has not been made explicit. All officers must prove to a court assessing a vehicle search is a reasonable ground for belief of guilt. Further, when a court is making a probable cause determination for itself in determining if a warrant should issue, it must decide only if there is a “fair probability that contraband or evidence of a crime will be found in a particular place.” What is a fair probability?
In the context of drug detection dogs, where we have actual data on reliability, assigning a numerical value to probable cause — or at least to the maximum false positive percentage upon which an officer can rely — would add much needed clarity to Fourth Amendment law. It also does not undermine police officers’ ability to use their intuition, because the event precipitating a search is not an officer’s informed judgment, but the alert from a dog.