Ecological Fallacy Goes to Court

In previous posts, I have bemoaned the judiciary’s tin ear for important qualitative differences between and among different research study designs.  The Reference Manual for Scientific Evidence (3d ed. 2011)(RMSE3d) offers inconsistent advice, ranging from Margaret Berger’s counsel to abandon any hierarchy of evidence, to other chapters’ emphasizing the importance of a hierarchy.

The Cook case is one of the more aberrant decisions, which elevated an ecological study, without a statistically significant result, into an acceptable basis for a causal conclusion under Rule 702.  Senior Judge Kane’s decision in the litigation over radioactive contamination from the Colorado Rocky Flats nuclear weapons plant is illustrative of a judicial refusal to engage with the substantive differences among studies, and to ignore the inability of some study designs to support causality.  See Cook v. Rockwell Internat’l Corp., 580 F. Supp. 2d 1071, 1097-98 (D. Colo. 2006) (“Defendants assert that ecological studies are inherently unreliable and therefore inadmissible under Rule 702.  Ecological studies, however, are one of several methods of epidemiological study that are well-recognized and accepted in the scientific community.”), rev’d and remanded on other grounds, 618 F.3d 1127 (10th Cir. 2010), cert. denied, ___ U.S. ___ (May 24, 2012).  Senior Judge Kane’s point about the recognition and acceptance of ecological studies has nothing to do with their ability to support conclusions of causality.  This basic non sequitur led the trial judge into ruling that the challenge “goes to the weight, not the admissibility” of the challenged opinion testimony.  This is a bit like using an election day exit poll, with 5% returns, for “reliable” evidence to support a prediction of the winner.  The poll may have been conducted most expertly, but it lacks the ability to predict the winner.

The issue is not whether ecological studies are “scientific”; they are part of the epidemiologists’ toolkit.  The issue is whether they warrant inferences of causation.  Some so-called scientific studies are merely hypothesis generating, preliminary, tentative, or data-dredging exercises.  Judge Kane opined that ecological studies are merely “less probative” than other studies, and the relative weights of studies do not render them inadmissible.  Id.  This is a misunderstanding or an abdication of gatekeeping responsibility.  First, studies themselves are not admissible; it is the expert witness, whose testimony is challenged.  Second, Rule 702 requires that the proffered opinion be “scientific knowledge,” and ecological studies simply lack the necessary epistemic warrant.

The legal sources cited by Senior Judge Kane provide only equivocal and minimal support at best for his decision.  The court pointed to RSME2d at 344-45, for the proposition that ecological studies are useful for establishing associations, but are weak evidence for causality. The other legal citations give seem equally unhelpful.  In re Hanford Nuclear Reservation Litig., No. CY–91– 3015–AAM, 1998 WL 775340 at *106 (E.D.Wash. Aug.21, 1998) (citing RMSE2d and the National Academy of Science Committee on Radiation Dose Reconstruction for Epidemiological Uses, which states that “ecological studies are usually regarded as hypothesis generating at best, and their results must be regarded as questionable until confirmed with cohort or case‑control studies.” National Research Council, Radiation Dose Reconstruction for Epidemiologic Uses at 70 (1995)), rev’d on other grounds, 292 F.3d 1124 (9th Cir. 2002).  Ruff v. Ensign– Bickford Indus., Inc., 168 F.Supp. 2d 1271, 1282 (D. Utah 2001) (reviewing evidence that consisted of a case-control study in addition to an ecological study; “It is well established in the scientific community that ecological studies are correlational studies and generally provide relatively weak evidence for establishing a conclusive cause and effect relationship.’’); see also id. at 1274 n.3 (“Ecological studies tend to be less reliable than case–control studies and are given little evidentiary weight with respect to establishing causation.”)



The new edition of RMSE cites the Cook case at several places.  In an introductory chapter, the late Professor Margaret Berger cites the case incorrectly for having excluded expert witness testimony.  See Margaret A. Berger, “The Admissibility of Expert Testimony 11, 24 n.62 in RMSE3d (“See Cook v. Rockwell Int’l Corp., 580 F. Supp. 2d 1071 (D. Colo. 2006) (discussing why the court excluded expert’s testimony, even though his epidemiological study did not produce statistically significant results).”)  The chapter on epidemiology cites Cook correctly for having refused to exclude the plaintiffs’ expert witness, Dr. Richard Clapp, who relied upon an ecological study of two cancer outcomes in the area adjacent to the Rocky Flats Nuclear Weapons Plant.  See Michael D. Green, D. Michal Freedman, and Leon Gordis, “Reference Guide on Epidemiology,” 549, 561 n. 34, in Reference Manual for Scientific Evidence (3d ed. 2011).  The authors, however, abstain from any judgmental comments about the Cook case, which is curious given their careful treatment of ecological studies and their limitations:

“4. Ecological studies

Up to now, we have discussed studies in which data on both exposure and health outcome are obtained for each individual included in the study.33 In contrast, studies that collect data only about the group as a whole are called ecological studies.34 In ecological studies, information about individuals is generally not gathered; instead, overall rates of disease or death for different groups are obtained and compared. The objective is to identify some difference between the two groups, such as diet, genetic makeup, or alcohol consumption, that might explain differences in the risk of disease observed in the two groups.35 Such studies may be useful for identifying associations, but they rarely provide definitive causal answers.36

Id. at 561.  The epidemiology chapter proceeds to note that the lack of information about individual exposure and disease outcome in an ecological study “detracts from the usefulness of the study,” and renders it prone to erroneous inferences about the association between exposure and outcome, “a problem known as an ecological fallacy.”  Id. at 562.  The chapter authors define the ecological fallacy:

“Also, aggregation bias, ecological bias. An error that occurs from inferring that a relationship that exists for groups is also true for individuals.  For example, if a country with a higher proportion of fishermen also has a higher rate of suicides, then inferring that fishermen must be more likely to commit suicide is an ecological fallacy.”

Id. at 623.  Although the ecological study design is weak and generally unsuitable to support causal inferences, the authors note that such studies can be useful in generating hypotheses for future research using studies that gather data about individuals. Id. at 562.  See also David Kaye & David Freedman, “Reference Guide on Statistics,” 211, 266 n.130 (citing the epidemiology chapter “for suggesting that ecological studies of exposure and disease are ‘far from conclusive’ because of the lack of data on confounding variables (a much more general problem) as well as the possible aggregation bias”); Leon Gordis, Epidemiology 205-06 (3d ed. 2004)(ecologic studies can be of value to suggest future research, but “[i]n and of themselves, however, they do not demonstrate conclusively that a causal association exists”).

The views expressed in the Reference Manual for Scientific Evidence, about ecological studies, are hardly unique.  The following quotes show how ecological studies are typically evaluated in epidemiology texts:

Ecological fallacy

An ecological fallacy or bias results if inappropriate conclusions are drawn on the basis of ecological data. The bias occurs because the association observed between variables at the group level does not necessarily represent the association that exists at the individual level (see Chapter 2).


Such ecological inferences, however limited, can provide a fruitful start for more detailed epidemiological work.”

R. Bonita, R. Beaglehole, and T. Kjellström, Basic Epidemiology 43 2d ed. (WHO 2006).

“A first observation of a presumed relationship between exposure and disease is often done at the group level by correlating one group characteristic with an outcome, i.e. in an attempt to relate differences in morbidity or mortality of population groups to differences in their local environment, living habits or other factors. Such correlational studies that are usually based on existing data are prone to the so-called ‘ecological fallacy’ since the compared populations may also differ in many other uncontrolled factors that are related to the disease. Nevertheless, ecological studies can provide clues to etiological hypotheses and may serve as a gateway towards more detailed investigations.”

Wolfgang Ahrens & Iris Pigeot, eds., Handbook of Epidemiology 17-18 (2005).

The Cook case is a wonderful illustration of the judicial mindset that avoids and evades gatekeeping by resorting to the conclusory reasoning that a challenge “goes to the weight, not the admissibility” of an expert witness’s opinion.

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