The Z line on the diagram represents the point at which the evidence is evenly balanced; that is, evidence of equal probative value supports the plaintiff’s position and the defendant’s. In a civil case the plaintiff’s burden of proof is to convince the trier of fact—usually the jury—that her version of events is more probably true than the defendant’s—that is, that a simple preponderance of the evidence favors the plaintiff’s version of events. In terms of the diagram, her evidence must fall to the right of the Z line to carry her burden of proof. The Y line represents the point at which the plaintiff’s proof becomes so strong that any reasonable jury would have to conclude that the plaintiff has proved her case.
Between the X and Y lines lies the arena for legitimate differences of opinion as to the proper outcome—and hence the arena for jury decision. Even though the judge concludes that the evidence falls at point Q, well to the left of the point where the balance of the evidence favors the plaintiff, she must allow the jury to decide the outcome. If the judge could take the case from the jury when the proof lies between X and Y, she could essentially usurp the role of the jury by substituting her own judgment for that of the jury at any time. General practice (and the constitutional right to jury trial) denies the judge such power: If reasonable minds can differ as to the result, the case is for the jury, not the judge.
Similarly, if the judge concludes that the balance of the evidence on an issue falls at point R on the spectrum, where the plaintiff’s evidence substantially out-weighs the opposing evidence (but is not so persuasive that the only reasonable verdict is for the plaintiff), the case must also go to the jury. Even though the judge believes that the balance tips in the plaintiff’s favor, at this point on the spectrum that conclusion is legitimately debatable, just as it is at point Q. If the jury could rationally find for the defendant, it must be given the opportunity to consider the case, even though the judge believes that the preponderance of the evidence favors the plaintiff.
The courts have long provided a procedural means for judges to take cases away from the jury if the plaintiff’s evidence does not reach the magical X line. If the judge concludes that the plaintiff’s case is so weak that no jury, acting rationally on the evidence before it, could find for her, allowing the case to go to the jury simply invites irrational decision-making based on irrelevant or prejudicial factors. The judge has traditionally had the authority to guard against such flawed verdicts by refusing to send cases to the jury if there is no legitimate doubt as to which side should prevail. See Rutherford v. Illlinois Central R.R., 278 F.2d 310, 312 (5th Cir.) cert. denied, 364 U.S. 922 (1960) (court’s power to take case from jury provides “a method for protecting neutral principles of law from powerful forces outside the scope of law—compassion and prejudice”).