In Kramer v. Lewisville Mem'l Hosp., 858 S.W.2d 397, 400 (Tex. 1993), the court explained, "the ultimate standard of proof on the causation issue" in a medical malpractice case is "whether, by a preponderance of the evidence, the negligent act or omission is shown to be a substantial factor in bringing about the harm and without which the harm would not have occurred." 858 S.W.2d at 400. The court continued, "the effect of these standards is to bar recovery where the defendant's negligence deprived the tort victim of only a 50% chance or less of avoiding the ultimate harm." Id.