The Cherry-Picking Fallacy in Synthesizing Evidence

What could be wrong with picking cherries?  At the end of the process you have cherries, and if you do it right, you have all ripe, and no rotten, cherries.  Your collection of ripe cherries, however, will be unrepresentative of the universe of cherries, but at least we understand how and why your cherries were selected.

Elite colleges pick the best high school students; leading law schools pick the top college students; and top law firms and federal judges cherry pick the best students of the best law schools.  Lawyers are all-too-comfortable with “cherry picking.”  Of course, the cherry-picking process here has at least some objective criteria, which can be stated in advance of the selection.

In litigation, each side is expected to “cherry pick” the favorable evidence, and ignore or flyblow the contrary evidence.  Judges are thus often complacent about selectivity in the presentation of evidence by parties and their witnesses.  In science, this kind of adversarial selectivity is a sure way to inject bias and subjectivity into claims of knowledge.  The development of the systematic review, in large measure, has been supported by the widespread recognition that studies cannot be evaluated with post hoc, subjective evaluative criteria. Cynthia D. Mulrow, Deborah J. Cook, Frank Davidoff, “Systematic Reviews: Critical Links in the Great Chain of Evidence,” 126 Ann. Intern. Med. 389 (1997)

The International Encyclopedia of Philosophy describes “cherry picking” as a fallacy, “a kind of error in reasoning.”  Cherry-picking the evidence, also known as “suppressed evidence,” is:

“[i]ntentionally failing to use information suspected of being relevant and significant is committing the fallacy of suppressed evidence. This fallacy usually occurs when the information counts against one’s own conclusion. * * * If the relevant information is not intentionally suppressed but rather inadvertently overlooked, the fallacy of suppressed evidence also is said to occur, although the fallacy’s name is misleading in this case.”

Bradley Dowden, “Suppressed Evidence,” International Encyclopedia of Philosophy (Last updated: December 31, 2010).

Cherry picking is a main rhetorical device for the litigator, and many judges simply do not understand what is so wrong with each side’s selection of the studies that it wishes to emphasize.  Whatever the acceptability of lawyers’ cherry picking in the presentation of evidence, it is antithetical to scientific methodology.  “Cherry picking (fallacy),” Wikipedia (describing cherry picking as the pointing to data that appears to confirm one’s opinion, while ignoring contradictory data)[last visited on June 14, 2012]

Given the pejorative connotations of “cherry picking,” no one should be very surprised that lawyers and judges couch their Rule 702 arguments and opinions in terms of whether expert witnesses engaged in this fruitful behavior.  Although I had heard plaintiffs’ and defendants’ counsel use the phrase, I only recently came across it in a judicial opinion.  Since the phrase nicely describes a fallacious form of reasoning, I thought it would be helpful to collect pertinent cases that describe the fallaciousness of fruit-pickin’ expert witness testimony.

United States Court of Appeals

Barber v. United Airlines, Inc., 17 Fed.Appx. 433, 437 (7th Cir. 2001) (affirming exclusion of “cherry-picking” expert witness who failed to explain why he ignored certain data while accepting others)

District Courts

Dwyer v. Sec’y of Health & Human Servs., No. 03-1202V, 2010 WL 892250 (Fed. Cl. Spec. Mstr. Mar. 12, 2010)(recommending rejection of thimerosal autism claim)(“In general, respondent’s experts provided more responsive answers to such questions.  Respondent’s experts were generally more careful and nuanced in their expert reports and testimony. In contrast, petitioners’ experts were more likely to offer opinions that exceeded their areas of expertise, to “cherry-pick” data from articles that were otherwise unsupportive of their position, or to draw conclusions unsupported by the data cited… .”)

In re Bausch & Lomb, Inc., 2009 WL 2750462 at 13 (D. S.C. 2009)( “Dr. Cohen did not address [four contradictory] studies in her expert reports or affidavit, and did not include them on her literature reviewed list [. . .] This failure to address this contrary data renders plaintiffs’ theory inherently unreliable.”)

Rimbert v. Eli Lilly & Co., No. 06-0874, 2009 WL 2208570, *19 (D.N.M. July 21, 2009) )(“Even more damaging . . . is her failure to grapple with any of the myriad epidemiological studies that refute her conclusion.”), aff’d, 647 F.3d 1247 (10th Cir. 2011) (affirming exclusion but remanding to permit plaintiff to find a new expert witness)

In re Bextra & Celebrex Prod. Liab. Litig., 524 F. Supp.2d 1166, 1176, 1179, 1181, 1184 (N.D. Cal. 2007) (criticizing plaintiffs’ expert witnesses for “cherry-picking studies”); id. at 1181 (“these experts ignore the great weight of the observational studies that contradict their conclusion and rely on the handful that appear to support their litigation-created opinion.”)

LeClerq v. Lockformer Co., No. 00 C 7164, 2005 U.S. Dist. LEXIS 7602, at *15 (N.D. Ill. Apr. 28, 2005) (holding that expert witness’s “cherry-pick[ing] the facts he considered to render his opinion, and such selective use of facts fail[s] to satisfy the scientific method and Daubert.”)(internal citations and quotations omitted)

Holden Metal & Aluminum Works v. Wismarq Corp., No. 00 C 0191, 2003 WL 1797844, at *2 (N.D. Ill. Apr. 2, 2003).

State Courts

Betz v. Pneumo Abex LLC, 2012 WL 1860853, *16 (May 23, 2012 Pa. S. Ct.)(“According to Appellants, moreover, the pathologist’s self-admitted selectivity in his approach to the literature is decidedly inconsistent with the scientific method. Accord Brief for Amici Scientists at 17 n.2 (“‘Cherry picking’ the literature is also a departure from ‘accepted procedure’.”)).

George v. Vermont League of Cities and Towns, 2010 Vt. 1, 993 A.2d 367, 398 (Vt. 2010)(expressing concern about how and why plaintiff’s expert witnesses selected some studies to include in their “weight of evidence” methodology.  Without an adequate explanation of selection and weighting criteria, the choices seemed arbitrary)

Scaife v. AstraZeneca LP, 2009 WL 1610575 at 8 (Del. Super. 2009) (“Simply stated, the expert cannot accept some but reject other data from the medical literature without explaining the bases for her acceptance or rejection.”)

In re Bextra & Celebrex, 2008 N.Y. Misc. LEXIS 720, *20, 239 N.Y.L.J. 27 (2008) (holding that New York’s Frye rule requires proponent to show that its expert witness had “look[ed] at the totality of the evidence and [did] not ignore contrary data.”); see also id. at *36 (“Moreover, out of 32 studies (29 published) cited by defendants, plaintiffs chose only 8 to plead their case.  This smacks of ‘cherry-picking,’ skewing their analysis by only looking at the helpful studies. Such practice contradicts the accepted method for an expert’s analysis of epidemiological data.”)

Bowen v. E.I. DuPont de Nemours & Co., 906 A.2d 787, 797 (Del. 2006) (noting that expert witnesses cannot ignore studies contrary to their opinions)

Selig v. Pfizer, Inc., 185 Misc. 2d 600, 607, 713 N.Y.S.2d 898 (Sup. Ct. N.Y. Cty. 2000) (holding that expert witness failed to satisfy Frye test’s requirement of following an accepted methodology when he ignored studies contrary to his opinion), aff’d, 290 A.D.2d 319, 735 N.Y.S.2d 549 (1st Dep’t 2002)


Most but not all the caselaw uniformly recognizes the fallacy for an expert witness to engage in ad hoc selectivity in addressing studies upon which to rely.  In the following two cases, the cherry-picking was identified, but acquiesced in by judges.

McClellan v. I-Flow Corp., 710 F. Supp. 2d 1092, 1114 (D. Ore. 2010)(discussing cherry picking but rejecting “document by document” review)(“Finally, defendants contend that plaintiffs’ experts employ unreliable methodologies by ‘cherry-picking’ facts from certain studies and asserting reliance on the ‘totality’ or ‘global gestalt of medical evidence’. Defendants argue that in  doing so, plaintiffs’ experts fail to ‘painstakingly’ link each piece of data to their conclusions or explain how the evidence supports their opinions.”)

United States v. Paracha, 2006 WL 12768 (S.D. N.Y. Jan. 3, 2006)(rejecting challenge to terrorism expert on grounds that he cherry picked evidence in conspiracy prosecution involving al Queda)

King v. Burlington No. Santa Fe Ry, ___N.W.2d___, 277 Neb. Reports 203, 234 (2009)(noting that the law does “not preclude a trial court from considering as part of its reliability inquiry whether an expert has cherry-picked a couple of supporting studies from an overwhelming contrary body of literature,” but ignoring the force of the fallacious expert witness testimony by noting that the questionable expert witness (Frank) had some studies that showed associations between exposure to diesel exhaust or benzene and multiple myeloma).

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