The Supreme Court has repeatedly admonished that the standard for probable cause is not “finely tuned” or capable of “precise definition or quantification into percentages.”Well that's not helpful. But yeah, it's a bit unfair to overly fault cops for not meeting a definition you can't define.
But, uh, what is probable cause. I'm telling you there's no answer. But a working definition I've used and cops will know ("reason to believe...") is actually not that good because "reason to believe" implies more than 50/50 chance. It's less than that! Dig this, from US v. Humphries (4th Cir. 2004):
Similarly, we have stated (United States v. Jones, 1994) that the probable-cause standard does not require that the officer's belief be more likely true than false.Well, damn. That was sort of news to me. (Which is why I'm posting this.)
So less than 50 percent is clear. And "reasonable suspicion" is clearly (though only partially) defined as "less than probable cause." So we're talking a pretty low bar here. But I mention in terms of the low "hit rate" cited in the DOJ Report. What's good enough? 10 percent? 25 percent? It depends. But if the hit rate gets anything close to this level, you can't argue there's prima fascia evidence of unconstitutional policing.
And the argument that arrests are bad because charges are dropped? It's absurd. Along with bureaucratic BS (prosecutors march to a much different drummer than cops), the standard for conviction is "beyond a reasonable doubt." The standard for arrest is "probable cause" (which isn't even "more likely true than false." So of course good legal arrests will be dropped.
On top of that, most low-level offenses are abated by arrest. You don't actually prosecute people for loitering and trespassing on a stoop. A loitering arrest isn't bad because it's not prosecuted. It's never prosecuted. And for such minor offenses, officer have pretty low motivation to write a good report, since it really just doesn't matter.
[In my intro classes, I just want students to know that "reasonable suspicion," based on Terry, is the legal standard to justify a stop and/or frisk; probable cause, based on the 4th Amendment, is the legal standard needed for a search or arrest. And even this is a tough sell.]
[Thanks to somebody else for all the research and some of the writing here]