Using Statistics to Determine Whether Causation Is Adequately Proven in Medical Malpractice Actions Involving Multiple Events Preceding the Injury

• Publication year: 2015
• Citation: Vol. 2015 No. 2
• Author: H. Thomas Watson and Peder K. Batalden

Using Statistics to Determine Whether Causation is Adequately Proven in Medical Malpractice Actions Involving Multiple Events Preceding the Injury

H. Thomas Watson and Peder K. Batalden

H. Thomas Watson is a partner at Horvitz & Levy LLP. He has served as appellate counsel in the California Supreme Court and Courts of Appeal, the Ninth Circuit, and other jurisdictions. He has extensive experience in health care law, as well as in other areas of substantive and procedural law.

Peder K. Batalden is a partner at Horvitz & Levy LLP, an appellate boutique. He has extensive appellate experience, particularly in the federal circuit courts. Mr. Batalden clerked for Judge Kermit Bye in the Eighth Circuit Court of Appeals, and is a co-author of the Rutter Group's treatise on handling civil appeals before the U.S. Court of Appeals for the Ninth Circuit. Mr. Batalden is a former chair of the State Bar of California's Standing Committee on the Federal Courts.

To prove medical malpractice liability, the plaintiff must establish through competent expert testimony that, to a reasonable degree of medical certainty, the plaintiff's injury was probably caused by the defendant's negligence. To have evidentiary weight, an expert's medical causation opinion must be supported by an adequate foundation demonstrating why the expert is reasonably certain that probable causation exists. This article discusses what this standard of proof means, how statistical analysis helps to determine when the standard for proving medical causation has been met, and how causation can be properly established in situations where multiple events precede the injury and each event in the sequence is dependent on the preceding event.¹

Reasonable Medical Probability

Medical malpractice is a form of professional negligence. Like any plaintiff suing in negligence, a plaintiff seeking to recover damages for medical malpractice must prove causation by a preponderance of the evidence. This requires a "show[ing] that defendants' breach of the standard of care was the cause, within a reasonable medical probability, of his injury."²

Although numerous published cases describe expert witnesses as testifying to a reasonable degree of medical certainty, showing a "reasonable medical probability" does not require proof of causation "to a 'medical certainty.'"³ Rather, a "reasonable medical probability" means there is at least a 51% chance that the defendant's breach of duty caused the injury, because a "possible cause only becomes 'probable' when, in the absence of other reasonable causal explanations, it becomes more likely than not that the injury was a result of [the defendant's] action."⁴ Proof "that there is some theoretical possibility the negligent act could have been a cause-in-fact of a particular injury is insufficient to establish causation."⁵

Reasonable medical certainty refers to the confidence level of the expert who is opining on the issue of whether a causal nexus between the alleged malpractice and the ensuing injury probably exists. It is a statistical term of art. Where there is very little data supporting an expert's opinion, the expert cannot have a high level confidence (i.e., a reasonable degree of medical certainty) that the data supporting an opinion of probable causation is not a fluke. But where there is an abundance of data, test results, and studies supporting the expert's opinion, the expert can testify with a high degree of confidence (i.e., a reasonable degree of medical certainty) regarding an opinion about probable causation.

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Since questions of medical causation generally exceed the common knowledge of lay jurors, "[t]he law is well settled that . . . causation must be proven within a reasonable medical probability based upon competent expert testimony."⁶ Of course, an expert witness "does not possess a carte blanche to express any opinion within the area of expertise."⁷ Experts must not assume facts lacking evidentiary support and must not unduly speculate or guess; instead, they must tether a case's "factual predicates" to their ultimate conclusion by "reasoned explanation," "because an 'expert opinion is worth no more than the reasons upon which it rests.'"⁸

It is vital to examine expert witnesses to determine whether their opinions regarding medical causation are supported by an adequate foundation. Given the legal requirement of proving causation by a reasonable medical probability, it is advisable for experts to testify about the relative certainty of their opinions. See, e.g., Maxwell v. Powers, 22 Cal. App. 4th 1596, 1601 (1994) ("[Dr.] Orloff opined there was a better than 90% probability, to a reasonable medical certainty, that the kidney could have been saved had kidney surgery been performed on the day Maxwell was admitted to Scripps Hospital."). If experts do not initially volunteer to state the relative certainty of their opinions when they testify, counsel should follow up with additional questions designed to elicit that information. If, for example, an expert were to testify that he is 100% certain that the negligent failure to administer a particular drug caused an adverse result—because the one time that drug had been administered previously no adverse result...