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<pre>                 United States Court of Appeals <br>                     For the First Circuit <br> <br> <br> <br> <br> <br>No. 98-1163 <br> <br>                 JULIO ELVIN RUIZ-TROCHE, ET AL., <br> <br>                      Plaintiffs, Appellees, <br> <br>                                v. <br> <br>       PEPSI COLA OF PUERTO RICO BOTTLING COMPANY, ET AL., <br> <br>                     Defendants, Appellants. <br> <br> <br> <br>           APPEAL FROM THE UNITED STATES DISTRICT COURT <br> <br>                 FOR THE DISTRICT OF PUERTO RICO <br> <br>       [Hon. Raymond L. Acosta, Senior U.S. District Judge] <br> <br> <br> <br>                              Before <br> <br>                      Selya, Circuit Judge, <br>                                 <br>                 Aldrich, Senior Circuit Judge, <br>                                 <br>                   and Boudin, Circuit Judge. <br>                                 <br> <br> <br>     Stephen A. Cozen with whom Elizabeth J. Chambers, Cozen and <br>O'Connor, Francisco J. Coln-Pagn, Francisco E. Coln-Ramrez, and <br>Coln, Coln & Martinez were on brief, for appellants. <br>     Jos F. Quetglas Jordan and Jorg Carazo-Quetglas, with whom <br>Eric M. Quetglas Jordan, Quetglas Law Offices, and Toledo Toledo & <br>Carazo-Quetglas, P.C. were on brief, for appellees. <br> <br> <br> <br> <br> <br>December 1, 1998 <br> <br> <br> <br>

  SELYA, Circuit Judge.  This appeal requires us to explore <br>the limits of a trial court's authority to exclude scientific <br>evidence   in this instance, evidence of alleged cocaine use by the <br>driver of a motor vehicle involved in a fatal accident and evidence <br>of his ensuing impairment   under Daubert v. Merrell Dow Pharm., <br>Inc., 509 U.S. 579 (1993).  We conclude that the court below abused <br>its discretion in excluding certain of this evidence and that the <br>magnitude of the error necessitates a new trial. <br>I.  BACKGROUND <br>  On an afternoon in September of 1992, Julio Elvin Ruiz <br>Cintrn (Ruiz) was driving a Toyota automobile westerly along a <br>two-lane road in Puerto Rico.  At what proved to be the critical <br>moment, Ruiz left his lane to pass a slow-moving vehicle.  Seconds <br>later, his car collided with an oncoming eastbound tractor-trailer <br>rig.  Ruiz, his wife, son, and three other passengers (all minors) <br>were killed.  Ruiz's four-year-old daughter survived, but sustained <br>permanent brain damage. <br>  In due course, Ruiz's daughter, joined by other relatives <br>of the various decedents (all of whom, at the time of suit, were <br>citizens of mainland states), invoked diversity jurisdiction, 28 <br>U.S.C.  1332(a), and brought suit for damages in Puerto Rico's <br>federal district court.  They named as defendants the driver of the <br>tractor-trailer unit, Juan Hernndez Rosario (Hernndez); his <br>employer, Los Vaqueros de Transporte y Carga; the consignor,  Pepsi <br>Cola of Puerto Rico Bottling Company; and several insurers.  The <br>plaintiffs averred (1) that Hernndez needlessly accelerated his <br>rig as the Toyota approached, thereby shortening the available time <br>within which Ruiz could complete his passing maneuver and return to <br>his own side of the road, and (2) that Hernndez refused to veer to <br>the right to avoid the accident, despite having sufficient space <br>and time to do so.  The defendants denied the essential allegations <br>of the complaint.  They argued that Ruiz, and Ruiz alone, had <br>caused the accident by recklessly initiating a passing maneuver in <br>the face of obvious danger and placing his vehicle on the wrong <br>side of the road.  To bolster this thesis, the defendants sought to <br>show that cocaine intoxication provoked Ruiz's recklessness. <br>  The district court stymied the defendants' anticipated <br>trial strategy by refusing to admit into evidence either the <br>toxicology section of the autopsy report (which reflected the <br>presence of cocaine and cocaine metabolites in Ruiz's bloodstream) <br>or expert testimony regarding the significance of these findings.  <br>In the court's view, the proposed expert testimony failed to meet <br>the standard of reliability required under Daubert.  Having <br>rejected the expert testimony, the court then excluded the <br>toxicology results under Fed. R. Evid. 403, concluding that those <br>results, standing alone and unexplained, were more prejudicial than <br>probative. <br>  In diversity cases, local law provides the substantive <br>rules of decision.  See Erie R.R. Co. v. Tompkins, 304 U.S. 64, 78 <br>(1938); Daigle v. Maine Med. Ctr., Inc., 14 F.3d 684, 689 (1st Cir. <br>1994).  Puerto Rico law recognizes comparative negligence <br>principles.  See P.R. Laws Ann. tit. 31,  5141 (1990).  The trial <br>judge instructed accordingly, and the jury found both drivers <br>negligent, assigning 59% of the fault to Ruiz and 41% to Hernndez.  <br>It then awarded damages totaling approximately $13,000,000 to the <br>various plaintiffs.  Due to Puerto Rico's combination of <br>comparative negligence and joint and several liability rules, the <br>defendants will be required to pay the full amount of these damages <br>if the judgment becomes final. <br>     In the aftermath of the jury verdict, the defendants <br>moved for judgment as a matter of law, Fed. R. Civ. P. 50, or, <br>alternatively, for a new trial, Fed. R. Civ. P. 59.  The district <br>court rejected the defendants' plaints, including those that <br>centered on the allegedly wrongful exclusion of the expert <br>testimony and toxicology results.  This appeal ensued. <br>II.  DISCUSSION <br>     The Daubert questions in this case are complex and <br>implicate four interrelated pieces of evidence:  (1) the toxicology <br>results contained in the autopsy report; (2) the so-called "dosage" <br>testimony, i.e., the expert opinions of a pharmacologist relating <br>to the amount of drugs that Ruiz consumed and the time of their <br>consumption, arrived at by interpolation from the toxicology <br>results; (3) the so-called "impairment" testimony, i.e., the <br>pharmacologist's expert opinions regarding the effects of cocaine <br>on behavior; and (4) the so-called "causation" testimony, i.e., <br>certain expert opinions of the defense's accident <br>reconstructionist.  After surveying the legal landscape, we discuss <br>how these items came before the district court and how the court <br>handled them.  We then explicate the standard of review and proceed <br>to test the correctness of the district court's rulings. <br>                      A.  Daubert Revisited. <br>     The Evidence Rules generally confine the testimony of a <br>lay witness to matters about which he or she has personal <br>knowledge, see Fed. R. Evid. 602, although such a witness may offer <br>opinions that are "rationally based on [his or her] perception" and <br>"helpful to a clear understanding of the witness' testimony or the <br>determination of a fact in issue," Fed. R. Evid. 701.  The Rules <br>afford expert witnesses much more leeway.  "If scientific, <br>technical, or other specialized knowledge will assist the trier of <br>fact to understand the evidence or determine a fact in issue, a <br>witness qualified as an expert by knowledge, skill, experience, <br>training, or education, may testify thereto in the form of an <br>opinion or otherwise."  Fed. R. Evid. 702.  Despite its apparent <br>breadth, this language does not give experts carte blanche, but, <br>rather, envisions some regulation of expert testimony by trial <br>judges.  The Court's opinion in Daubert furnishes the principal <br>source of guidance on the proper fulfillment of this gatekeeping <br>role. <br>     We start with an historical perspective.  Prior to <br>Daubert, courts and commentators regarded Frye v. United States, <br>293 F. 1013 (D.C. Cir. 1923), as the watershed case on the <br>admission of expert opinion testimony.  Under Frye, the <br>admissibility of an expert opinion or technique turned on its <br>"general acceptance" vel non within the scientific community.  Id.at 1014.  Daubert tackled the question of whether the Frye standard <br>survived the passage of the Federal Rules of Evidence (and, in <br>particular, Rule 702) and answered that question in the negative, <br>holding that Rule 702 displaced the Frye test.  See Daubert, 509 <br>U.S. at 587-89. <br>     The Daubert Court's interpretation of Rule 702, drawn <br>from its text, requires the trial judge to evaluate an expert's <br>proposed testimony for both reliability and relevance prior to <br>admitting it.  See id. at 589-95.  The requisite review for <br>reliability includes consideration of several factors:  the <br>verifiability of the expert's theory or technique, the error rate <br>inherent therein, whether the theory or technique has been <br>published and/or subjected to peer review, and its level of <br>acceptance within the scientific community.  See id. at 593-95.  <br>The Court reasoned that due investigation of such matters will <br>ensure that proposed expert testimony imparts "scientific <br>knowledge" rather than guesswork.  Id. at 592.  Withal, the factors <br>that the Court enumerated do not function as a "definitive <br>checklist or test," but form the basis for a flexible inquiry into <br>the overall reliability of a proffered expert's methodology.  Id.at 593. <br>     Along with the reliability requirement, the Daubert Court <br>imposed a special relevancy requirement.  See id. at 591-92.  To be <br>admissible, expert testimony must be relevant not only in the sense <br>that all evidence must be relevant, see Fed. R. Evid. 402, but also <br>in the incremental sense that the expert's proposed opinion, if <br>admitted, likely would assist the trier of fact to understand or <br>determine a fact in issue, see Daubert, 509 U.S. at 591-92.  In <br>other words, Rule 702, as visualized through the Daubert prism, <br>"requires a valid scientific connection to the pertinent inquiry as <br>a precondition to admissibility."  Id. at 592. <br>     In General Elec. Co. v. Joiner, 118 S. Ct. 512 (1997), <br>the Justices established the appropriate standard of appellate <br>review for Daubert determinations, concluding that a reviewing <br>tribunal should scrutinize a trial court's decision to allow or <br>disallow the admission of expert testimony on Daubert grounds for <br>abuse of discretion.  See id. at 517.  Joiner also placed a gloss <br>on Daubert's insistence that trial courts focus on an expert's <br>methodology, rather than his conclusions, in order to determine the <br>reliability of his testimony.  See Daubert, 509 U.S. at 595.  The <br>Joiner Court moderated this position, acknowledging that <br>     conclusions and methodology are not entirely <br>     distinct from one another.  Trained experts <br>     commonly extrapolate from existing data.  But <br>     nothing in either Daubert or the Federal Rules <br>     of Evidence requires a district court to admit <br>     opinion evidence which is connected to <br>     existing data only by the ipse dixit of the <br>     expert.  A court may conclude that there is <br>     simply too great an analytical gap between the <br>     data and the opinion proffered. <br> <br>Joiner, 118 S. Ct. at 519.  Thus, while methodology remains the <br>central focus of a Daubert inquiry, this focus need not completely <br>pretermit judicial consideration of an expert's conclusions.  <br>Rather, trial judges may evaluate the data offered to support an <br>expert's bottom-line opinions to determine if that data provides <br>adequate support to mark the expert's testimony as reliable. <br>     Daubert and Joiner, though critically important, do not <br>represent the sum total of available jurisprudential insights.  <br>Since Daubert hove into view, the courts of appeals have made <br>significant contributions to an understanding of how to separate <br>reliable from unreliable science and how to apply the intuitive <br>idea of "fit"   as courts have come to call the special kind of <br>relevance that Daubert demands   to live litigation scenarios.  <br>See, e.g., Baker v. Dalkon Shield Claimants Trust, 156 F.3d 248, <br>252-54 (1st Cir. 1998); Daubert v. Merrell Dow Pharm., Inc., 43 <br>F.3d 1311, 1316-22 (9th Cir. 1995).  Nonetheless, choreographing <br>the Daubert pavane remains an exceedingly difficult task.  Few <br>federal judges are scientists, and none are trained in even a <br>fraction of the many scientific fields in which experts may seek to <br>testify.  Moreover, even though Daubert and its progeny require <br>trial judges to evaluate the level of support provided by complex <br>scientific studies and experiments in myriad disciplines, <br>reliability and relevance remain legal judgments.  Trial judges <br>cannot abdicate the responsibility for making those judgments by <br>delegating them to the scientific community. <br>     To complicate matters further, Daubert issues rarely <br>arise in a vacuum, but, rather, frequently collide in practice with <br>the requirements of other rules of evidence, especially Fed. R. <br>Evid. 403.  This phenomenon adds yet another dimension to the <br>decisional calculus.  See, e.g., Baker, 156 F.3d at 254. <br>                   B.  The Challenged Evidence. <br>     We turn now from the general to the particular, and <br>canvass the evidence tendered below insofar as it bears on this <br>appeal.  Following Ruiz's death, the authorities ordered an <br>autopsy.  The autopsy report included toxicology results indicating <br>that Ruiz's body contained 0.45 mcg/ml of cocaine and 0.15 mcg/ml <br>of a cocaine metabolite (benzoylecgonine) in its blood, cocaine <br>metabolites in its urine and vitreous humor, and cocaine in its <br>nasal passages.  The defendants proffered Dr. James O'Donnell, a <br>well-credentialed pharmacologist, to comment upon the significance <br>of these findings.  Dr. O'Donnell was prepared to testify, based on <br>the autopsy report, that in his opinion Ruiz had snorted 200 <br>milligrams of cocaine within an hour prior to the accident.  Dr. <br>O'Donnell also proposed to testify that cocaine impairs senses and <br>capabilities affecting driving, diminishes perception, and <br>increases the willingness to take risks.  The district court <br>excluded not only Dr. O'Donnell's testimony, but also the <br>toxicology results.  And, though the court permitted the <br>defendants' accident reconstruction expert, Eric Cintrn, to <br>testify, the court apparently precluded counsel from eliciting his <br>opinion that Ruiz would not have initiated the fateful passing <br>maneuver had it not been for his cocaine intoxication. <br> <br>                      C.  Proceedings Below. <br>     The evidentiary issues in this case came to the fore on <br>February 28, 1997, when the plaintiffs filed motions in limine <br>seeking the exclusion of both Dr. O'Donnell's testimony and the <br>toxicology results.  The plaintiffs argued that levels of cocaine <br>and metabolites in the bloodstream could not be correlated to <br>initial dosage or levels of impairment with the degree of certainty <br>required for scientific evidence, and they produced several <br>articles to this effect.  They sought exclusion of the toxicology <br>results mainly on the theory that the chain of custody of the <br>samples of bodily fluids had been compromised.  The defendants <br>promptly submitted an opposition.  They proffered Dr. O'Donnell's <br>report, the report of the plaintiffs' expert toxicologist <br>(suggesting that he used a methodology similar to that employed by <br>Dr. O'Donnell), and portions of plaintiffs' expert's deposition.  <br>Based on these materials, they argued that Dr. O'Donnell's <br>methodology was sufficiently reliable to withstand Daubertscrutiny.  As to the toxicology results, the defendants contended <br>that the plaintiffs' argument was factually wrong, and that, in all <br>events, any gaps in the chain of custody went to the weight of the <br>evidence, not to its admissibility.  See, e.g., United States v. <br>Ladd, 885 F.2d 954, 956 (1st Cir. 1989). <br>     The district court held a pretrial conference on March 5, <br>1997.  At the court's request, defense counsel agreed to furnish <br>copies of the 18 articles cited by Dr. O'Donnell in his report, and <br>submitted 14 of them within the next few days.  Without waiting for <br>the remainder, the district court ruled on March 10.  It concluded <br>that there was "no scientific basis" for Dr. O'Donnell's dosage <br>opinion (i.e., his opinion that Ruiz had snorted at least 200 mg of <br>powdered cocaine within an hour before the accident) and that Dr. <br>O'Donnell's impairment opinion (i.e., his opinion that cocaine <br>ingestion had impaired Ruiz's driving ability) likewise failed to <br>pass the Daubert reliability screen.  To close the circle, the <br>court barred introduction of the toxicology results "in light of" <br>the exclusion of Dr. O'Donnell's testimony. <br>     Trial began the next day.  On March 26, the plaintiffs <br>moved in limine to exclude the testimony of Eric Cintrn.  Noting <br>a statement in Cintrn's report to the effect that Ruiz "appears to <br>have been driving under the influence of drugs as it is evidenced <br>by the toxicological report," and his conclusion that "this may <br>explain the reason for [the] . . . passing maneuver," the <br>plaintiffs argued for exclusion both because the trial court had <br>banished the toxicology results and because Cintrn was unqualified <br>to opine on the relationship between cocaine and driving ability.  <br>Although the district court never formally ruled on this motion, <br>Cintrn was not asked his opinion on these matters when he <br>testified, and the parties treat this omission as flowing from the <br>district court's prohibition on any mention of the toxicology <br>results. <br>     On March 31   the same day that the plaintiffs completed <br>their case in chief   the defendants filed a motion to admit Dr. <br>O'Donnell's testimony and attached to it copies of numerous <br>additional articles and portions of learned treatises regarding <br>cocaine's absorption, excretion, detection in bodily fluids, and <br>effects on the senses and behavior.  Judge Acosta denied the motion <br>the following day, terming it "untimely."  Other motions for <br>reconsideration, made at divers points during the presentation of <br>the defense's case, met a similar fate. <br>                     D.  Standard of Review. <br>     Nobody questions that testimony regarding dosage   what <br>quantity of drugs Ruiz consumed, and when   and the effect of <br>Ruiz's cocaine consumption on his driving ability would be relevant <br>as long as it could be reliably determined that Ruiz ingested an <br>appreciable amount of cocaine shortly before the accident and that <br>such dosage probably would impair one's ability to drive.  Thus, <br>although the Daubert standard requires that expert testimony be <br>both reliable and relevant as a precondition to admissibility, the <br>reliability inquiry is central in this case. <br>     The district court concluded that Dr. O'Donnell's <br>testimony on both dosage and impairment lacked scientific <br>reliability and precluded the testimony on that basis.  Forbidding <br>the toxicology results and Cintrn's opinion on causation followed <br>directly from this exclusionary ruling.  We review all these <br>determinations for abuse of discretion.  See Joiner, 118 S. Ct. at <br>517. <br>     While this standard of review ordinarily is "not <br>appellant-friendly,"  Lussier v. Runyon, 50 F.3d 1103, 1111 (1st <br>Cir. 1995), it does not render trial court decisions impervious to <br>scrutiny.  On abuse-of-discretion review, we will reverse a trial <br>court's decision if we determine that the judge "committed a <br>meaningful error in judgment."  Anderson v. Cryovac, Inc., 862 F.2d <br>910, 923 (1st Cir. 1988); see also Foster v. Mydas Assocs., Inc., <br>943 F.2d 139, 143 (1st Cir. 1991) (explaining that an abuse of <br>discretion occurs "when a material factor deserving significant <br>weight is ignored, when an improper factor is relied upon, or when <br>all proper and no improper factors are assessed, but the court <br>makes a serious mistake in weighing them") (quoting Independent Oil <br>& Chem. Workers of Quincy, Inc. v. Procter & Gamble Mfg. Co., 864 <br>F.2d 927, 929 (1st Cir. 1988)). <br>                          E.  Analysis. <br>     To determine whether the nisi prius court committed "a <br>meaningful error in judgment" here, we assay its evaluation of each <br>contested piece of evidence under the Daubert standard. <br>     1.  The Dosage Testimony.  We first consider the <br>exclusion of Dr. O'Donnell's testimony concerning the approximate <br>time of Ruiz's cocaine consumption and the amount of cocaine <br>actually ingested by him.  Based on autopsy data   cocaine and <br>cocaine metabolites found in Ruiz's blood, urine, nasal passages, <br>and vitreous humor   and half-life values for the substances <br>involved, Dr. O'Donnell applied a mathematical formula and <br>concluded "with reasonable pharmacological certainty" that Ruiz <br>"'snorted' at least 200-mg of cocaine powder into his nose thirty <br>(30) to sixty (60) minutes prior to the accident."  Although Dr. <br>O'Donnell's report cites numerous scientific writings in support of <br>the methodology underlying this proposition, the lower court found <br>none of these sources adequate to imbue the proffered opinions with <br>the patina of reliability required by Daubert. <br>     We have read the articles mentioned by Dr. O'Donnell and <br>supplied to the district court.  Several of them discuss a half- <br>life for cocaine or its metabolites and/or the times after <br>ingestion at which cocaine and its metabolites reach peak <br>concentrations in the body.  One of these sources is a standard <br>medical textbook.  See Matthew J. Ellenhorn & Donald G. Barceloux, <br>Medical Toxicology 648-49 (1988) (placing half-life in blood plasma <br>for cocaine administered intransally at 1.3 hours and citing the <br>average half-life for such cocaine in urine at 75 minutes).  <br>Another is a published article in a prestigious, peer-reviewed <br>medical journal.  See Peter M. Marzuk, et al., Fatal Injuries After <br>Cocaine Use As a Leading Cause of Death Among Young Adults in New <br>York City, 332 New Eng. J. Med. 1753, 1754 (1995) (using half-life <br>of cocaine metabolites in urine to determine use of cocaine by <br>victims of fatal car accidents).  The publication of these pieces <br>and their exposure to peer review serve as independent indicia of <br>the reliability of the half-life technique.  By the same token, <br>publication and peer review also demonstrate a measure of <br>acceptance of the methodology within the scientific community. <br>     Secondary sources cited by Dr. O'Donnell lack publication <br>and peer review, see, e.g., Daniel S. Isenschmid, Cocaine (Plasma <br>Concentrations of Cocaine Metabolites in Humans) 11-12 (collecting <br>and reporting studies finding half-lives for cocaine metabolites in <br>urine and peak levels of cocaine in plasma after intranasal <br>administration of cocaine), but this circumstance does not make <br>such sources per se unacceptable, see Daubert, 509 U.S. at 593 <br>(explaining that neither publication nor peer review is "a sine quanon of admissibility").  Under ordinary circumstances, an <br>unpublished, unreviewed work, standing alone, probably would be <br>insufficient to demonstrate the reliability of a scientific <br>technique.  But when such an article makes the same point as <br>published, peer-reviewed pieces, it tends to strengthen the <br>assessment of reliability. <br>     Other works referenced by Dr. O'Donnell detail controlled <br>studies correlating dosages with levels of cocaine (and its <br>metabolites) remaining in the body after certain periods of time.  <br>See, e.g., Randall C. Baselt & Robert H. Cravey, Disposition of <br>Toxic Drugs and Chemicals in Man 208-09 (1989) (collecting <br>studies); H.E. Hamilton, et al., Cocaine and Benzoylecgonine <br>Excretion in Humans, 22 J. Forensic Sci. 697, 698-706 (1987) <br>(reporting results of a study that administered cocaine <br>intranasally to healthy subjects and tested urine for levels of <br>cocaine and its metabolites after 1, 2, 4, 8, 12, 24, 48, 72, 120, <br>144, and 168 hours).  These manuscripts further confirm the <br>reliability of Dr. O'Donnell's approach by providing information <br>that can be used to test the accuracy of the technique upon which <br>he relied.  And, finally, the fact that the plaintiffs' expert <br>employed essentially the same technique furnishes added validation. <br>     To be sure, the scientific literature does not make out <br>an open-and-shut case.  In defense of the trial court's ruling, the <br>plaintiffs point to statements (some of which appear in works <br>referenced by Dr. O'Donnell) that the half-life of cocaine varies <br>among individuals due to many factors, and that cocaine metabolites <br>can be found in the body days after ingestion.  See, e.g., <br>Ellenhorn & Barceloux, supra at 649 (stating that the half-life of <br>cocaine in urine "varies significantly between individuals"); <br>Hamilton, et al., supra, at 703 (finding cocaine metabolites in <br>subjects' urine up to 120 hours after insufflation of cocaine).  <br>The plaintiffs posit that these statements indicate the inherent <br>unreliability of Dr. O'Donnell's dosage opinion.  If half-life <br>varies, they ruminate, it cannot be used to determine initial <br>dosage with any accuracy   and the discovery of metabolites offers <br>no more information because these may remain in the body long after <br>the drug's effects on behavior have subsided.  Thus, the plaintiffs <br>asseverate, Daubert forbids the admission of Dr. O'Donnell's <br>testimony. <br>     We think that the plaintiffs (and the district court) set <br>the bar too high.  Although the statements that they assemble cast <br>doubt on Dr. O'Donnell's position   for example, those statements <br>suggest that the half-life technique for calculating dosage has an <br>uncertain rate of error   no single factor disposes of a <br>reliability inquiry.  See Daubert, 509 U.S. at 592-95.  Dr. <br>O'Donnell's technique has been subjected to, and survived, the <br>rigors of testing, publication, and peer review, and it appears to <br>have won significant (if not universal) acceptance within the <br>scientific community.  Daubert does not require that a party who <br>proffers expert testimony carry the burden of proving to the judge <br>that the expert's assessment of the situation is correct.  As long <br>as an expert's scientific testimony rests upon "good grounds, based <br>on what is known," Daubert, 509 U.S. at 590 (internal quotation <br>marks omitted), it should be tested by the adversary process   <br>competing expert testimony and active cross-examination   rather <br>than excluded from jurors' scrutiny for fear that they will not <br>grasp its complexities or satisfactorily weigh its inadequacies, <br>see id. at 596.  In short, Daubert neither requires nor empowers <br>trial courts to determine which of several competing scientific <br>theories has the best provenance.  It demands only that the <br>proponent of the evidence show that the expert's conclusion has <br>been arrived at in a scientifically sound and methodologically <br>reliable fashion.  See Kannankeril v. Terminix Int'l, Inc., 128 <br>F.3d 802, 806 (3d Cir. 1997); In re Paoli R.R. Yard PCB Litig., 35 <br>F.3d 717, 744 (3d Cir. 1994). <br>     On balance, we find that Dr. O'Donnell's dosage opinion, <br>incorporating a range of time in which he believed Ruiz took the <br>cocaine, satisfies this standard.  The opinion was premised on an <br>accepted technique, embodied a methodology that has significant <br>support in the relevant universe of scientific literature, and was <br>expressed to a reasonable degree of pharmacological certainty.  <br>While the literature does not irrefutably prove the accuracy of Dr. <br>O'Donnell's dosage conclusions, it furnishes a sufficient <br>underpinning for those conclusions to forfend preclusion of his <br>testimony as unreliable.  Thus, the district court's refusal to <br>entertain Dr. O'Donnell's dosage opinion constituted an abuse of <br>discretion.  See Baker, 156 F.3d at 252-54; see also Ed Peters <br>Jewelry Co. v. C & J Jewelry Co., 124 F.3d 252, 258-61 (1st Cir. <br>1997) (finding, prior to Joiner, exclusion of expert testimony <br>erroneous under either abuse of discretion or a more stringent <br>standard of review). <br>     2.  The Impairment Testimony.  Although Dr. O'Donnell's <br>opinion on dosage contained sufficient indicia of reliability to <br>warrant its admission into evidence, the defendants needed other <br>expert evidence to flesh out their theory as to how the accident <br>occurred.  In particular, they had to show a correlation between <br>cocaine use in the dosage suggested by Dr. O'Donnell's opinion and <br>an impairment affecting the cocaine user's fitness to drive.  Dr. <br>O'Donnell was ready to supply the missing link:  he opined that <br>cocaine intoxication resulting from a dose such as Ruiz insufflated <br>results in impairments of perception, reflexes, reaction time, and <br>judgment, and that such a degree of intoxication increases one's <br>"sense of mastery" and thus promotes risk taking.  Virtually all of <br>these sequelae would adversely affect the ability to drive safely. <br>     The plaintiffs urged the trial court to hold that Dr. <br>O'Donnell's impairment testimony failed Daubert's reliability prong <br>because science cannot correlate particular impairments to cocaine <br>concentrations within the body.  See, e.g., Peter M. Marzuk, et <br>al., Prevalence of Recent Cocaine Use Among Motor Vehicle <br>Fatalities in New York City, 253 J. Am. Med. Ass'n 250, 255 (1990).  <br>In the plaintiffs' view, only an immediate neurological examination <br>could have provided sufficiently reliable evidence as to whether <br>Ruiz suffered from cocaine intoxication at the time of the accident <br>(and if so, to what extent)   and no such examination was <br>performed. <br>     The court granted the plaintiffs' motion to exclude the <br>impairment testimony.  We quote the core of the court's reasoning, <br>as explicated in its subsequent denial of the defendants' motion <br>for a new trial: <br>     All the scientific literature submitted to the <br>     Court on this issue unanimously concluded that <br>     unlike alcohol, a correlation between a <br>     particular amount of cocaine in the system and <br>     the degree of impairment produced has not been <br>     scientifically determined because the effects <br>     of this substance var[y] from one individual <br>     to another.  Thus, from the scientific <br>     evidence reviewed by the Court it was <br>     concluded that even though it is generally <br>     accepted that cocaine causes impairment in an <br>     individual [it] cannot be scientifically <br>     deduced from the amount found in his or her <br>     system because persons metabolize cocaine <br>     differently. <br> <br>     We believe that the court's rationale conflated the <br>dosage and impairment issues.  The court agreed that "cocaine <br>causes impairment," but rejected Dr. O'Donnell's proposed testimony <br>on this point because of its distrust of the witness's dosage <br>testimony.  By relying so heavily on an improper factor in the <br>decisional calculus   the dosage testimony, as we have said, was <br>sufficiently reliable to satisfy Daubert   the district court <br>abused its discretion.  See Foster, 943 F.2d at 143; Independent <br>Oil Workers, 864 F.2d at 929. <br>     To compound this error, the court applied a standard of <br>scientific certainty to the impairment testimony beyond that which <br>Daubert envisions.  The court imposed a threshold requirement that <br>science be able to declare that a precise quantity of cocaine in <br>the bloodstream produces an equally precise degree of impairment.  <br>This requirement solicits a level of assurance that science <br>realistically cannot achieve and that Daubert does not demand.  SeeDaubert, 509 U.S. at 590 (commenting that "arguably, there are no <br>certainties in science").  The adoption of such a standard <br>impermissibly changes the trial judge's role under Daubert from <br>that of gatekeeper to that of armed guard.  That mistaken <br>application of the law likewise constitutes an abuse of discretion.  <br>See United States v. Snyder, 136 F.3d 65, 67 (1st Cir. 1998) <br>(holding that a per se abuse of discretion occurs when a district <br>court commits an error of law). <br>     3.  The Remaining Evidence.  Having decided to ban Dr. <br>O'Donnell's proffered testimony, the district court held that the <br>prejudicial effect of admitting the toxicology results would <br>substantially outweigh their probative value and excluded this <br>evidence under Rule 403.  This is perfectly understandable:  to <br>allow the jury to be told that traces of cocaine were found in <br>Ruiz's body without any accompanying explanation of the meaning of <br>the test results or of cocaine's capacity to impair driving skills <br>would sow the seeds for an horrific harvest.  It is evident, <br>however, that the lower court's rulings regarding the admissibility <br>of Dr. O'Donnell's proffered testimony influenced the calibration <br>of the Rule 403 balance.  The error inherent in those rulings <br>requires vacation of this aspect of the district court's order vis- <br>-vis the toxicology results. <br>     The district court's prohibition of Cintrn's causation <br>opinion also must be reconsidered.  Cintrn's proposed testimony as <br>to cocaine's role in the accident was based on other evidence <br>(e.g., the toxicology results) that the district court erroneously <br>excluded.  This bevue nullifies the most obvious reason for <br>precluding Cintrn from testifying anent causation.  As with the <br>toxicology results, this ruling will have to be revisited by the <br>district court on remand. <br>                    F.  Effect of the Errors. <br>     Our odyssey is not yet concluded.  The plaintiffs contend <br>that even if the trial court erred in excluding evidence anent <br>Ruiz's use of cocaine, the mistakes are harmless and do not require <br>a new trial.  See Fed. R. Civ. P. 61; Fed. R. Evid. 103. <br>     The plaintiffs' argument is not without some force.  <br>Puerto Rico is a comparative negligence jurisdiction that imposes <br>joint and several liability on joint tortfeasors.  This doctrinal <br>combination means that a driver whose negligence is found to have <br>contributed to causing an accident may be solely responsible for <br>compensating the victims even though the other driver involved in <br>the accident was more negligent.  See generally Ramos Acosta, 116 <br>P.R. Offic. Trans. at 81-82. <br>     In this case, the district court properly directed the <br>jury to make findings anent comparative negligence, and the jury <br>assigned fault to both drivers (59% to Ruiz, 41% to Hernndez).  <br>The plaintiffs maintain that the evidence of cocaine use was <br>relevant only to Ruiz's negligence and that its prohibitation did <br>not undermine the jury's finding that Hernndez's negligence <br>contributed significantly to the accident's occurrence.  Since <br>Hernndez's negligence subjects the defendants to liability for the <br>full amount of the verdict, see supra note 1, the plaintiffs <br>hypothesize that the improper exclusion of the cocaine evidence <br>does not require a new trial.  We reject this hypothesis. <br>     The draconian potential of the Puerto Rico rules is <br>mitigated by the theories of efficient cause and absorption.  Under <br>the principle of efficient cause, when one party is the "sole, and <br>efficient cause of the damage," another's negligence during the <br>accident, if it did not cause it, does not subject him to <br>liability.  Toro Lugo v. Ortiz Martnez, 113 P.R. Offic. Trans. 73, <br>75, 113 P.R. Dec. 56, 56 (1982).  Under the absorption theory, if <br>one tortfeasor is only slightly responsible, the overwhelming <br>negligence of the other tortfeasor "absorbs" the minimal negligence <br>of the former and the latter bears all liability.  See id.; seealso Santiago v. Becton Dickinson & Co., 571 F. Supp. 904, 911-14 <br>(D.P.R. 1983) (comparing efficient cause and absorption <br>principles). <br>     These theories have particular pertinence here. Ruiz's <br>alleged cocaine intoxication formed the foundation not only for the <br>defendants' insistence that Ruiz was guilty of negligence, but also <br>for their insistence that Hernndez was not negligent.  With the <br>benefit of the cocaine evidence, the jurors may well have balanced <br>the proofs of negligence quite differently.  After all, they found <br>Ruiz 59% negligent even without the damaging evidence suggesting <br>that his judgment and driving ability may have been impaired by <br>cocaine at the time of the crash.  The additional evidence, if <br>ultimately shown to be admissible, easily could lead a rational <br>jury to find Ruiz's negligence to have been so great as to <br>overwhelm Hernndez's negligence. <br>     An erroneous evidentiary ruling requires vacation of a <br>jury verdict if the ruling excludes evidence and "the exclusion <br>results in actual prejudice because it had a substantial and <br>injurious effect or influence in determining the jury's verdict."  <br>United States v. Shay, 57 F.3d 126, 134 (1st Cir. 1995) (citation <br>and internal quotation marks omitted).  In order to determine <br>whether a particular ruling had a sufficiently pernicious effect on <br>a verdict, a reviewing court must resist the temptation to deal in <br>abstractions, and must mull the ruling in context, giving due <br>weight to the totality of the relevant circumstances.  See Nieves- <br>Villanueva v. Soto-Rivera, 133 F.3d 92, 102 (1st Cir. 1997).  To <br>sustain the verdict, the reviewing court must be able to say with <br>a fair degree of assurance that the erroneous ruling did not <br>substantially sway the jury.  See Ladd, 885 F.2d at 957 (citing <br>Kotteakos v. United States, 328 U.S. 750, 765 (1946)).  We can <br>muster no such assurance here. <br>     We reach this conclusion despite the somewhat awkward <br>procedural posture in which this appeal arises.  In the <br>prototypical "exclusion of evidence" case, the court of appeals <br>assesses the effect that a piece of evidence, wrongly withheld from <br>the jury, likely would have had if introduced.  See, e.g., <br>Espeaignnette v. Gene Tierney Co., 43 F.3d 1, 5-9 (1st Cir. 1994).  <br>This case is different.  Even with the misplaced Daubert obstacle <br>removed, the dosage testimony is not necessarily admissible.  <br>Rather, its admissibility depends to some extent upon whether <br>certain other evidence, specifically, the toxicology results and <br>the impairment testimony, prove to be admissible.  Without the <br>former (to establish the basis for Dr. O'Donnell's dosage opinion) <br>and the latter (to explain the probable effects of Ruiz's cocaine <br>use on his driving), the dosage testimony, though deemed reliable <br>in the Daubert sense, might not be admissible in the long run.  <br>Yet, when a trial court erroneously excludes evidence, and the <br>exclusion meets the standard criteria of harmfulness, the harm is <br>not cured by a mere possibility that other appropriate grounds for <br>exclusion of the same evidence may later be found to exist.  The <br>question is one of degree and the choice of remedies (including <br>whether to require a new trial or merely remand for further <br>findings) is ours.  See 28 U.S.C.  2106. <br>     In this instance, we think that the defendants' chances <br>of succeeding in their effort to introduce the cocaine-related <br>evidence are promising enough that the improper exclusion of the <br>dosage testimony, coupled with the errors regarding the court's <br>consideration of the impairment testimony, can be said to have <br>materially curtailed the defendants' opportunity to present their <br>theory of the case to the jury.  This, in turn, worked a <br>substantial and injurious effect on the jury's ability to evaluate <br>liability.  Taking into account all aspects of the situation, we <br>are persuaded that vacation of the judgment, rather than a remand <br>for further findings, is the fairest course.  A new trial will <br>allow a judge appropriately to ascertain the admissibility of <br>expert testimony and a jury armed with all reliable and relevant <br>evidence to weigh issues of comparative fault on a scale that is in <br>balance.  To this end, we reject the plaintiffs' claim that any <br>error committed by the trial court was benign. <br>III.  CONCLUSION <br>     We need go no further.  By excluding Dr. O'Donnell's <br>dosage testimony, the district court exceeded the scope of its <br>discretion under Daubert.  This fundamental error infected certain <br>other evidentiary rulings.  On the facts of this case, there is too <br>great a risk that these rulings, in cumulation, had a substantial <br>and injurious influence upon the jury's determinations.  Hence, we <br>reverse the district court's Daubert ruling, vacate the judgment <br>below, and order a new trial.  We emphasize that, in doing so, we <br>hold only that the dosage testimony is sufficiently reliable to be <br>admissible.  We do not pass upon the ultimate admissibility of that <br>evidence (which, as we have said, depends upon the admissibility of <br>the toxicology results, see supra note 7, and the admissibility of <br>the correlative impairment testimony).  By like token, we do not <br>pass upon the ultimate admissibility of either the impairment or <br>causation testimony.  That evidence must be reexamined within the <br>limits dictated by Daubert and its progeny. <br> <br>Reversed and remanded for a new trial.</pre>

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