"Shaky" Expert Witness Opinions May Not Tank a Case

Expert witness opinions claiming that the Roundup weed killer causes cancer are “shaky”, U.S. District Court Judge Vince Chhabria concluded at the end of multiple Daubert hearings on March 13th, leaving the case against Monsanto on shaky grounds as well. But the plaintiff's only need to get one expert through the Daubert door to keep the case going.

Justice Chhabria heard from nearly a dozen experts, toxicologists, statisticians, an oncologist and two epidemiologists. Chhabria said he had a “difficult time understanding how an epidemiologist in the face of all the evidence” that was presented in court the previous week could conclude the the herbicide “is in fact causing” cancer (non-Hodgkin lympoma) in human beings. The evidence, Chhabria said, is “pretty sparse.”

Chhabria noted though that it wasn't his role to decide on causation but to determine whether the expert witnesseses were qualified. The Justice took particular interest in the two opposing epidemiologists as the case rests heavily on their testimony.

Monsanto's epidemiologist expert witness, Harvard University professor Lorelei Mucci, relied heavily on the National Institute for Health's Agricultural Health Study which has been tracking the health of some 54,000 farmers in Iowa and South Carolina over decades. Professor Mucci, claims that the study, most recently updated in 2017, is the “most powerful scientific evidence” that no link exists between Glyphosate and cancer.

Most of the plaintiff's experts relied heavily on an assessment that Glyphosate is probably a human carcinogen made by the International Agency for Research on Cancer (IARC), an arm of the World Health Organization, in 2015.

The IARC expert's review included the U.S. Agricultural Health Study, as well as other government reports and peer-reviewed studies, although under its standard rules it did reject a number of industry-submitted studies. Evidence linking Glyphosate to tumors in mice and rats and mechanistic evidence (eg., DNA damage to human cells from exposure to glyphosate), led the IARC to its conclusion that Glyphosate is “probably carcinogenic.” The IARC's review was later summarized in The Lancet, one of the most highly regarded peer-reviewed general medical journals.

Chhabria rejected those experts who based their opinions on the IARC study, concluding that it's “not enough” to claim exposure to glyphosate is more likely than not the cause of the plaintiffs' cancer.

The plaintiff's epidemiologist, University of California at Los Angeles public health professor, Beate Ritz, did not rely on the IARC review but did her own research emphasizing the importance of using control groups (those not exposed to glyphosate or free of cancer) as comparisons. Her report included a map of the U.S. use of glyphosate in 1994 (two years before Monsanto introduced its Roundoup Ready product) and in 2014, after the explosive growth had led to almost every square mile of Iowa being blanketed by more than 88 pounds of glyphosate a year. With so much glyphosate in use, Ritz argued, it's impossible to accurately measure exposure levels among Iowa's farmers, and thus impossible to know glyphosate's effects on cancer risk based on the Agricultual Health Study.

After the hearings, Justice Chhabria concluded that epidemiology is a “loosey-goosey” and “highly subjective field.” Due to constraints on his ability to eliminate witnesses, that may leave room for Ritz to testify, Chhabria admitted. Ritz may, he said, be “operating within the mainstream of the field.” Chhabria allowed that he may have to leave it up to the jury “to decide if they buy her presentation,” admitting that his own math skills “are less than rudimentary,” which would make understanding the statistics behind the epidemiology studies challenging.

While some experts might expect a scientifically valid report to be sufficient, Justice Chhabria's remarks reenforce the old truth that experts have to be capable of “selling” their presentations, both to judge and jury.