Systematic Reviews and Meta-Analyses in Litigation

Kathy Batty is a bellwether plaintiff in a multi-district litigation[1] (MDL) against Zimmer, Inc., in which hundreds of plaintiffs claim that Zimmer’s NexGen Flex implants are prone to have their femoral and tibial elements prematurely aseptically loosen (independent of any infection). Batty v. Zimmer, Inc., MDL No. 2272, Master Docket No. 11 C 5468, No. 12 C 6279, 2015 WL 5050214 (N.D. Ill. Aug. 25, 2015) [cited as Batty].

PRISMA Guidelines for Systematic Reviews

Zimmer proffered Dr. Michael G. Vitale, an orthopedic surgeon, with a master’s degree in public health, to testify that, in his opinion, Batty’s causal claims were unfounded. Batty at *4. Dr. Vitale prepared a Rule 26 report that presented a formal, systematic review of the pertinent literature. Batty at *3. Plaintiff Batty challenged the admissibility of Dr. Vitale’s opinion on grounds that his purportedly “formal systematic literature review,” done for litigation, was biased and unreliable, and not conducted according to generally accepted principles for such reviews. The challenged was framed, cleverly, in terms of Dr. Vitale’s failure to comply with a published set of principles outlined in “PRISMA” guidelines (Preferred Reporting Items for Systematic reviews and Meta-Analyses), which enjoy widespread general acceptance among the clinical journals. See David Moher , Alessandro Liberati, Jennifer Tetzlaff, Douglas G. Altman, & The PRISMA Group, “Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement,” 6 PLoS Med e1000097 (2009) [PRISMA]. Batty at *5. The trial judge, Hon. Rebecca R. Pallmeyer, denied plaintiff’s motion to exclude Dr. Vitale, but in doing so accepted, arguendo, the plaintiff’s implicit premise that an expert witness’s opinion should be reached in the manner of a carefully constructed systematic review.

The plaintiff’s invocation of the PRISMA guidelines presented several difficult problems for her challenge and for the court. PRISMA provides a checklist of 27 items for journal editors to assess the quality and completeness of systematic reviews that are submitted for publication. Plaintiff Batty focused on several claimed deviations from the guidelines:

  • “failing to explicitly state his study question,
  • failing to acknowledge the limitations of his review,
  • failing to present his findings graphically, and failing to reproduce his search results.”

Batty’s challenge to Dr. Vitale thus turned on whether Zimmer’s expert witness had failed to deploy “same level of intellectual rigor,” as someone in the world of clinical medicine would [should] have in conducting a similar systematic review. Batty at *6.

Zimmer deflected the challenge, in part by arguing that PRISMA’s guidelines are for the reporting of systematic reviews, and they are not necessarily criteria for valid reviews. The trial court accepted this rebuttal, Batty at *7, but missed the point that some of the guidelines call for methods that are essential for rigorous, systematic reviews, in any forum, and do not merely specify “publishability.” To be sure, PRISMA itself does not always distinguish between what is essential for journal publication, as opposed to what is needed for a sufficiently valid systematic review. The guidelines, for instance, call for graphical displays, but in litigation, charts, graphs, and other demonstratives are often not produced until the eve of trial, when case management orders call for the parties to exchange such materials. In any event, Dr. Vitale’s omission of graphical representations of his findings was consistent with his finding that the studies were too clinical heterogeneous in study design, follow-up time, and pre-specified outcomes, to permit nice, graphical summaries. Batty at *7-8.

Similarly, the PRISMA guidelines call for a careful specification of the clinical question to be answered, but in litigation, the plaintiff’s causal claims frame the issue to be addressed by the defense expert witness’s literature review. The trial court readily found that Dr. Vitale’s research question was easily discerned from the context of his report in the particular litigation. Batty at *7.

Plaintiff Batty’s challenge pointed to Dr. Vitale’s failure to acknowledge explicitly the limitations of his systematic review, an omission that virtually defines expert witness reports in litigation. Given the availability of discovery tools, such as a deposition of Dr. Vitale (at which he readily conceded the limitations of his review), and the right of confrontation and cross-examination (which are not available, alas, for published articles), the trial court found that this alleged deviation was not particularly relevant to the plaintiff’s Rule 702 challenge. Batty at *8.

Batty further charged that Dr. Vitale had not “reproduced” his own systematic review. Arguing that a systematic review’s results must be “transparent and reproducible,” Batty claimed that Zimmer’s expert witness’s failure to compile a list of studies that were originally retrieved from his literature search deprived her, and the trial court, of the ability to determine whether the search was complete and unbiased. Batty at *8. Dr. Vitale’s search protocol and inclusionary and exclusionary criteria were, however, stated, explained, and reproducible, even though Dr. Vitale did not explain the application of his criteria to each individual published paper. In the final analysis, the trial court was unmoved by Batty’s critique, especially given that her expert witnesses failed to identify any relevant studies omitted from Dr. Vitale’s review. Batty at *8.

Lumping or Commingling of Heterogeneous Studies

The plaintiff pointed to Dr. Vitale’s “commingling” of studies, heterogeneous in terms of “study length, follow-up, size, design, power, outcome, range of motion, component type” and other clinical features, as a deep flaw in the challenged expert witness’s methodology. Batty at *9. Batty’s own retained expert witness, Dr. Kocher, supported Batty’s charge by adverting to the clinical variability in studies included in Dr. Vitale’s review, and suggesting that “[h]igh levels of heterogeneity preclude combining study results and making conclusions based on combining studies.” Dr. Kocher’s argument was rather beside the point because Dr. Vitale had not impermissibly combined clinically or statistically heterogeneous outcomes.[2] Similarly, the plaintiff’s complaint that Dr. Vitale had used inconsistent criteria of knee implant survival rates was dismissed by the trial court, which easily found Dr. Vitale’s survival criteria both pre-specified and consistent across his review of studies, and relevant to the specific alleged by Ms. Batty. Batty at *9.

Cherry Picking

The trial court readily agreed with Plaintiff’s premise that an expert witness who used inconsistent inclusionary and exclusionary criteria would have to be excluded under Rule 702. Batty at *10, citing In Re Zoloft, 26 F. Supp. 3d 449, 460–61 (E.D. Pa.2014) (excluding epidemiologist Dr. Anick Bérard proffered testimony because of her biased cherry picking and selection of studies to support her studies, and her failure to account for contradictory evidence). The trial court, however, did find that Dr. Vitale’s review was corrupted by the kind of biased cherry picking that Judge Rufe found to have been committed by Dr. Anick Bérard, in the Zoloft MDL.

Duplicitous Duplication

Plaintiff’s challenge of Dr. Vitale did manage to spotlight an error in Dr. Vitale’s inclusion of two studies that were duplicate analyses of the same cohort. Apparently, Dr. Vitale had confused the studies as not being of the same cohort because the two papers reported different sample sizes. Dr. Vitale admitted that his double counting the same cohort “got by the peer-review process and it got by my filter as well.” Batty at *11, citing Vitale Dep. 284:3–12. The trial court judged Dr. Vitale’s error to have been:

“an inadvertent oversight, not an attempt to distort the data. It is also easily correctable by removing one of the studies from the Group 1 analysis so that instead of 28 out of 35 studies reporting 100% survival rates, only 27 out of 34 do so.”

Batty at *11.

The error of double counting studies in quantitative reviews and meta-analyses has become a prevalent problem in both published studies[3] and in litigation reports. Epidemiologic studies are sometimes updated and extended with additional follow up. The prohibition against double counting data is so obvious that it often is not even identified on checklists, such as PRISMA. Furthermore, double counting of studies, or subgroups within studies, is a flaw that most careful readers can identify in a meta-analysis, without advance training. According to statistician Stephen Senn, double counting of evidence is a serious problem in published meta-analytical studies.[4] Senn observes that he had little difficulty in finding examples of meta-analyses gone wrong, including meta-analyses with double counting of studies or data, in some of the leading clinical medical journals. Senn urges analysts to “[b]e vigilant about double counting,” and recommends that journals should withdraw meta-analyses promptly when mistakes are found.”[5]

An expert witness who wished to skate over the replication and consistency requirement might be tempted, as was Dr. Michael Freeman, to count the earlier and later iteration of the same basic study to count as “replication.” Proper methodology, however, prohibits double dipping data to count the later study that subsumes the early one as a “replication”:

“Generally accepted methodology considers statistically significant replication of study results in different populations because apparent associations may reflect flaws in methodology. Dr. Freeman claims the Alwan and Reefhuis studies demonstrate replication. However, the population Alwan studied is only a subset of the Reefhuis population and therefore they are effectively the same.”

Porter v. SmithKline Beecham Corp., No. 03275, 2015 WL 5970639, at *9 (Phila. Cty. Pennsylvania, Ct. C.P. October 5, 2015) (Mark I. Bernstein, J.)


The PRISMA and similar guidelines do not necessarily map the requisites of admissible expert witness opinion testimony, but they are a source of some important considerations for the validity of any conclusion about causality. On the other hand, by specifying the requisites of a good publication, some PRISMA guidelines are irrelevant to litigation reports and testimony of expert witnesses. Although Plaintiff Batty’s challenge overreached and failed, the premise of her challenge is noteworthy, as is the trial court’s having taken the premise seriously. Ultimately, the challenge to Dr. Vitale’s opinion failed because the specified PRISMA guidelines, supposedly violated, were either irrelevant or satisfied.

[1] Zimmer Nexgen Knee Implant Products Liability Litigation.

[2] Dr. Vitale’s review is thus easily distinguished from what has become commonplace in litigation of birth defect claims, where, for instance, some well-known statisticians [names available upon request] have conducted qualitative reviews and quantitative meta-analyses of highly disparate outcomes, such as any and all cardiovascular congenital anomalies. In one such case, a statistician expert witness hired by plaintiffs presented a meta-analysis that included study results of any nervous system defect, and central nervous system defect, and any neural tube defect, without any consideration of clinical heterogeneity or even overlap with study results.

[3] See, e.g., Shekoufeh Nikfar, Roja Rahimi, Narjes Hendoiee, and Mohammad Abdollahi, “Increasing the risk of spontaneous abortion and major malformations in newborns following use of serotonin reuptake inhibitors during pregnancy: A systematic review and updated meta-analysis,” 20 DARU J. Pharm. Sci. 75 (2012); Roja Rahimi, Shekoufeh Nikfara, Mohammad Abdollahic, “Pregnancy outcomes following exposure to serotonin reuptake inhibitors: a meta-analysis of clinical trials,” 22 Reproductive Toxicol. 571 (2006); Anick Bérard, Noha Iessa, Sonia Chaabane, Flory T. Muanda, Takoua Boukhris, and Jin-Ping Zhao, “The risk of major cardiac malformations associated with paroxetine use during the first trimester of pregnancy: A systematic review and meta-analysis,” 81 Brit. J. Clin. Pharmacol. (2016), in press, available at doi: 10.1111/bcp.12849.

[4] Stephen J. Senn, “Overstating the evidence – double counting in meta-analysis and related problems,” 9, at *1 BMC Medical Research Methodology 10 (2009).

[5] Id. at *1, *4.


Some papers and textbooks, in addition to Stephen Senn’s paper, cited above, which note the impermissible method of double counting data or studies in quantitative reviews.

Aaron Blair, Jeanne Burg, Jeffrey Foran, Herman, Gibb, Sander Greenland, Robert Morris, Gerhard Raabe, David Savitz, Jane Teta, Dan Wartenberg, Otto Wong, and Rae Zimmerman, “Guidelines for Application of Meta-analysis in Environmental Epidemiology,” 22 Regulatory Toxicol. & Pharmacol. 189, 190 (1995).

“II. Desirable and Undesirable Attributes of Meta-Analysis

* * *

Redundant information: When more than one study has been conducted on the same cohort, the later or updated version should be included and the earlier study excluded, provided that later versions supply adequate information for the meta-analysis. Exclusion of, or in rare cases, carefully adjusting for overlapping or duplicated studies will prevent overweighting of the results by one study. This is a critical issue where the same cohort is reexamined or updated several times. Where duplication exists, decision criteria should be developed to determine which of the studies are to be included and which excluded.”

Sander Greenland & Keith O’Rourke, “Meta-Analysis – Chapter 33,” in Kenneth J. Rothman, Sander Greenland, Timothy L. Lash, Modern Epidemiology 652, 655 (3d ed. 2008) (emphasis added)

Conducting a Sound and Credible Meta-Analysis

Like any scientific study, an ideal meta-analysis would follow an explicit protocol that is fully replicable by others. This ideal can be hard to attain, but meeting certain conditions can enhance soundness (validity) and credibility (believability). Among these conditions we include the following:

  • A clearly defined set of research questions to address.

  • An explicit and detailed working protocol.

  • A replicable literature-search strategy.

  • Explicit study inclusion and exclusion criteria, with a rationale for each.

  • Nonoverlap of included studies (use of separate subjects in different included studies), or use of statistical methods that account for overlap.* * * * *”

Matthias Egger, George Davey Smith, and Douglas G. Altman, Systematic Reviews in Health Care: Meta-Analysis in Context 59 – 60 (2001).

Duplicate (multiple) publication bias


The production of multiple publications from single studies can lead to bias in a number of ways.85 Most importantly, studies with significant results are more likely to lead to multiple publications and presentations,45 which makes it more likely that they will be located and included in a meta-analysis. The inclusion of duplicated data may therefore lead to overestimation of treatment effects, as recently demonstrated for trials of the efficacy of ondansetron to prevent postoperative nausea and vomiting86.”

Khalid Khan, Regina Kunz, Joseph Kleijnen, and Gerd Antesp, Systematic Reviews to Support Evidence-Based Medicine: How to Review and Apply Findings of Healthcare Research 35 (2d ed. 2011)

“2.3.5 Selecting studies with duplicate publication

Reviewers often encounter multiple publications of the same study. Sometimes these will be exact  duplications, but at other times they might be serial publications with the more recent papers reporting increasing numbers of participants or lengths of follow-up. Inclusion of duplicated data would inevitably bias the data synthesis in the review, particularly because studies with more positive results are more likely to be duplicated. However, the examination of multiple reports of the same study may provide useful information about its quality and other characteristics not captured by a single report. Therefore, all such reports should be examined. However, the data should only be counted once using the largest, most complete report with the longest follow-up.”

Julia H. Littell, Jacqueline Corcoran, and Vijayan Pillai, Systematic Reviews and Meta-Analysis 62-63 (2008)

Duplicate and Multiple Reports


It is a bit more difficult to identify multiple reports that emanate from a single study. Sometimes these reports will have the same authors, sample sizes, program descriptions, and methodological details. However, author lines and sample sizes may vary, especially when there are reports on subsamples taken from the original study (e.g., preliminary results or special reports). Care must be taken to ensure that we know which reports are based on the same samples or on overlapping samples—in meta-analysis these should be considered multiple reports from a single study. When there are multiple reports on a single study, we put all of the citations for that study together in summary information on the study.”

Kay Dickersin, “Publication Bias: Recognizing the Problem, Understanding Its Origins and Scope, and Preventing Harm,” Chapter 2, in Hannah R. Rothstein, Alexander J. Sutton & Michael Borenstein, Publication Bias in Meta-Analysis – Prevention, Assessment and Adjustments 11, 26 (2005)

“Positive results appear to be published more often in duplicate, which can lead to overestimates of a treatment effect (Timmer et al., 2002).”

Julian P.T. Higgins & Sally Green, eds., Cochrane Handbook for Systematic Reviews of Interventions 152 (2008)

“7.2.2 Identifying multiple reports from the same study

Duplicate publication can introduce substantial biases if studies are  inadvertently included more than once in a meta-analysis (Tramer 1997). Duplicate publication can take various forms, ranging from identical manuscripts to reports describing different numbers of participants and different outcomes (von Elm 2004). It can be difficult to detect duplicate publication, and some ‘detectivework’ by the review authors may be required.”

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