Class #2 will be devoted to “The Blue Bus Problem” from the law of evidence. Harvard law professor Larry Tribe formulated this famous proof paradox over five decades ago as follows: “Plaintiff is negligently run down by a blue bus. The question is whether the bus belonged to the defendant. Plaintiff is prepared to prove that defendant operates four-fifths [or 80%] of all the blue buses in town. What effect, if any, should such proof be given?” Let’s call Larry Tribe’s hypo “Scenario A.” (This scenario is based on an actual case, Smith v. Rapid Transit, and is so famous in legal circles that it has generated numerous law review articles and taken up countless minutes in law school classrooms. I have previously surveyed this literature here.)
What if, however, the plaintiff is able to produce a witness, and what if the witness is prepared to testify that it was, in fact, the defendant’s blue bus that caused the accident? But what if the plaintiff’s testimony is reliable only 80% of the time? Let’s call this second hypo “Scenario B.” Either way, since the plaintiff in both scenarios is suing the defendant bus company for money damages (as is the case in most business disputes), the plaintiff is required to prove her case under “the preponderance of the evidence” standard. In plain English, in order to meet this standard, the plaintiff alleging a particular fact must prove that it is “more likely than not” that the fact is true.
Given this standard of proof, should the court rule for the plaintiff, or should it dismiss the case? Scenario A consists purely of statistical evidence or “naked probabilistic evidence” (e.g. the fraction of blue buses owned by the defendant), while Scenario B involves qualitative or “individualized evidence” in the form of eyewitness testimony, but the probabilities of accuracy are the same in both scenarios!