See also: CHILD ABUSE–Expert Testimony
IDENTIFICATIONS–Expert Testimony on Reliability; OPINIONS
PUBLICATIONS–Learned Treatises; SCIENTIFIC EVIDENCE; VALUATION–Expert Testimony

1.  Generally

Expert testimony is allowed where the testimony is relevant,{footnote}Cook v. American S.S. Co., 53 F.3d 733 (6th Cir. 1995).{/footnote} the witness is qualified in a particular field or subject matter, the testimony is based on a reliable principles or methods,{footnote}American & Foreign Ins. Co. v. General Electric Co., 45 F.3d 135 (6th Cir. 1995) (expert testimony properly excluded as unreliable where expert had established no protocol for test, had taken no notes during testing, and was unsure whether his equipment had been calibrated).
{/footnote} and the witness has applied the principles and methods reliably to the facts of the case.{footnote}FRE 702.{/footnote}  Whether to admit expert testimony is a matter within the trial court’s discretion.{footnote}United States v. Serna, 799 F.2d 842, 850 (2d Cir. 1986), cert. denied, 481 U.S. 1013 (1987); Marx & Co., Inc. v. Diners’ Club, Inc., 550 F.2d 505, 508 (2d Cir. 1977); Leonard v. Uniroyal, Inc., 765 F.2d 560, 566 (6th Cir. 1985); Unites States v. Purham, 725 F.2d 450, 454 (8th Cir. 1984); United States v. Brown, 540 F.2d 1048, 1053 (10th Cir. 1976), cert. denied, 429 U.S. 1100 (1977); Sanchez v. Safeway Stores, Inc. 451 F.2d 998 (10th Cir. 1971).
Johnson v. Lynch, 574 A.2d 934, 939 (N.H. 1990); Howerton v. Arai Helmet, 158 N.C. App. 316, 333, 581 S.E.2d 816, 827 (2003).
See also Fox v. Dannenberg, 906 F.2d 1253 (8th Cir. 1990) (doubts about whether an expert’s testimony will be useful should generally be resolved in favor of admissibility);{/footnote} 

Relevance
Experts may only testify as to matters as to which their testimonty would be helpful to the trier of fact.{footnote}Daubert v. Merrell Dow Pharmeceuticals, 43 F.3d 1311 (9th Cir. 1995) (expert testimony excluded as not helpful because expert could not say that chemical caused birth defect but only that it was “capable of causing” birth defect); Taylor v. Illinois Central Railroad, 8 F.3d 584 (7th Cir. 1993) (expert’s opinion that a pile of larger rocks is more difficult to stand on than a pile of smaller rocks was excluded as not helpful).{/footnote}  Text Box: FRE 702 If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

2.  Qualifying an Expert

Expert opinion testimony is admissible whenever the expert’s experience, training or knowledge renders his or her opinion helpful to the trier of fact in evaluating the evidence.{footnote}FRE 702; United States v. Fosher, 590 F.2d 381 (1st Cir. 1979); United States v. Onumonu, 967 F.2d 782 (2d Cir. 1992) (expert testimony regarding smuggling diamonds through use of internal caririers was improperly excluded as unhelpful). Knight v. Otis Elevator Co., 596 F.2d 84 (3d Cir. 1979) DeLuca v. Merrell Dow Pharmaceuticals, Inc., 911 F.2d 941, 956 (3d Cir. 1990) (Federal Rules of Evidence "embody a strong and undeniable preference for admitting any evidence having some potential for assisting the trier of fact and for dealing with the risk of error through the adversary process."); Carroll v. Otis Elev. Co., 896 F.2d 210 (7th Cir. 1990) (expert testimony that the color red attracts children, that a covered stop button is less accessible to children than an uncovered one, andthat the more difficult a button is to push, the less readily it is activated by a small child was properly admitted).
People v. Crooks, 250 Cal. App. 2d 788 (1967); Leonard v. Pistick Dairy, 464 N.E.2d 644 (Ill. App. Ct. 1984) (following FRE 702); Madrid v. Robinson, 931 P.2d 791 (Or. 1997).
See also Peters v. Five Star Marine Serv., 898 F.2d 448 (5th Cir. 1990) (maritime expert properly excluded from testifying that it was unreasonable for an employer to instruct his employee to manually move equipment on the deck of a boat during heavy seas, and that spilled diesel fuel had made the deck slippery); United States v. Esch, 832 F.2d 531(10th Cir. 1987) (generalized testimony by clinical psychologist concerningdefendant’s psychological characteristics held properly excluded in childpornography case); Green v. Kinney Shoe Corp., 715 F. Supp. 1122 (D.D.C. 1989) (expert testimony that a subjective promoting process could be used to hide discriminatory intent was not helpful).{/footnote}    A witness’ qualifications as an expert, if disputed, must be established to the court’s satisfaction by a preponderance of the evidence.  Many courts will allow opposing counsel to cross-examine the witness as to his or her qualifications before any substantive testimony is given.{footnote}United States v. York, 933 F.2d 1343 (7th Cir.), cert. denied, 112 S. Ct. 321 (1991) (the government’s voir dire in the jury’s presence focusing on prior unprofessional conduct of an expert pathologist was not improper because the defendant had qualified the witness before the jury, the questions were relevant to the weight to be accorded the expert’s testimony, the prosecutor had a good faith basis for the questions, and qualification is a preliminary question to which FRE 608(b) does not apply).{/footnote} See also REPUTATION–Experts.  Whether a witness is qualified as an expert is left to the sound discretion of the trial court.{footnote}Commonwealth v. Fryar, 680 N.E.2d 901 (Mass. 1997) (police laboratory chemist qualified to testify to blood spattering); Couch v. City of D’Iberville, 656 So.2d 146,152 (Miss. 1995); McBride v. State, 862 S.W.2d 600, 607-08 (Tex.Cr.App.), cert. denied, 512 U.S. 1246 (1993); Billiter v. Melton Truck Lines, Inc., 420 S.E.2d 286, 291 (W. Va. 1992).{/footnote}  Experts generally cannot be disqualified because of potential bias.{footnote}Snyder v. Whittaker Corp., 839 F.2d 1085 (5th Cir. 1988) (witness is not to be disqualified merely because he is a "professional expert," especially since this fact can be brought out on cross-examination); Gideon v. Johns-Manville Sales Corp., 761 F.2d 1129 (5th Cir. 1985) (expert in asbestosis action properly permitted to testify even though he had seen more than 600 asbestosis patients, most of whom had been referred by the plaintiff’s lawyers); Hingson v. Pacific Sw. Airlines, 743 F.2d 1408 (9th Cir. 1984) (that expert worked for defendant airline did not disqualify him from testifying for plaintiff).
{/footnote}  Some courts have even allowed parties to testify as experts on their own behalf.{footnote}Rodriguez v. Pacificare of Tex., Inc., 980 F.2d 1014 (5th Cir.), cert. denied, 113 S. Ct. 2456 (1993) (no abuse of discretion in accepting the party-physician’s affidavit as an expert in his own case, where the relevant qualifying criteria were satisfied); Tagatz v. Marquette Univ., 861 F.2d 1040 (7th Cir. 1988) (plaintiff permitted to testify as expert concerning his economic losses).
{/footnote}

Once the witness has been found qualified to testify as an expert, any deficiencies in the witness’ skill or knowledge go to the weight of the testimony, not its admissibility.{footnote}Knight v. Otis Elev., 596 F.2d 84 (3d Cir. 1979) (consulting chemist and engineer was qualified to render opinion concerning design and manufacture of elevators; inexperience in these areas goes to weight, not admissibility); Ellis v. K-Lan Co., 695 F.2d 157 (5th Cir. 1983) (expert’s lack of familiarity with a statutory standard affects the weight and not the admissibility of his testimony); Williams v. Pro-Tec, Inc., 908 F.2d 345 (8th Cir. 1990) (testimony by mechanical engineer that eyeguard would protect eyes from injury held properly admitted; lack of medical expertise went to the weight rather than to the admissibility of his opinion); United States v. Gwaltney, 790 F.2d 1378, 1382 (9th Cir. 1986), cert. denied, 479 U.S. 1104 (1988).
Commonwealth v. Yameen, 401 Mass. 331, 336 (1987), cert. denied, 486 U.S. 1008 (1988); {/footnote}  Even when deemd qualified, experts may only testify as to matters within their area of expertise.{footnote}Murray v. Florida, No. 83-556 (Fla. Supr. 4/17/97) (expert witness may not testify to matters that fall outside area of expertise); Broussard v. Huffman Mfg., 438 N.E.2d 1217 (Ill. App. Ct. 1982).{/footnote}

2(a).  Factors to Be Considered

The fundamental basis for qualifying an expert has been summed up in one word: experience.{footnote}Marx & Co. v. Diners’ Club, Inc., 550 F.2d 505, 512 (2d Cir. 1977), quoting Wigmore sec. 555.{/footnote}  Other factors include the witness’ education,{footnote}Friendship Heights Assocs. v. Vlastimil Koubek, A.I.A., 785 F.2d 1154 (4th Cir. 1986) (In an action by a building owner who claimed damages after paint peeled from a building soon after it was painted, it was reversible error to exclude as unqualified an expert witness who had a master’s degree in chemical and ceramic engineering and a doctorate in silicate sciences, as well as experience heading a laboratory that performed tests to determine the properties of building materials and concrete, even though the witness lacked practical experience).
{/footnote} training, familiarity with one or more published authorities in the area, and membership in professional associations.  It has also been held that the trial court must satisfy itself that the expert is being as careful as he or she would be in regular professional work outside paid litigation consulting.{footnote}Sheehan v. Daily Racing Form, Inc.,  __ F.3d __ (7th Cir. 1997); Thomas J. Kline, Inc. v. Lorillard, Inc., 878 F.2d 791 (4th Cir. 1989) (witness called as an “expert" to testify that a certain credit practice was tantamount to price discrimination was disqualified because her only stated qualification was that she was employed by a company who hired out experts to testify in sophisticated financial litigation).{/footnote}

2(b).  Medical Experts

Factors to be considered in determining whether a witness is qualified to testify as to the standard of care in a medical malpractice suit include the relationship between the witness’ specialty and the medical issue in question, amount of time devoted to the area, whether the witness is licensed in the same profession, and whether, in a case against a non-specialist, the witness is familiar with the standard of care practiced in the state.{footnote}Ill. sec. 8-2501.  But see Slezak v. Girzadas, 167 Ill. App. 3d 1045, 522 N.E.2d 132 (1st Dist. 1988) (no familiarity with local statndard of care required where there is relatively uniform statndard of care nationwide).
.{/footnote} 

2(c).  Law Enforcement Personnel

Experienced law enforcement agents may identify a substance as a specific narcotic or other drug,{footnote}United States v. Bermudez, 526 F.2d 89 (2d Cir. 1975) (cocaine).{/footnote} testify as to how a certain criminal scheme generally operates,{footnote}Moore v. United States, 394 F.2d 818 (5th Cir. 1968) (numbers racket).{/footnote} or testify regarding the cause of an accident.{footnote}Gladhill v. General Motors Corp., 743 F.2d 1049 (4th Cir.1984) (police officer who arrived at accident scene was qualified to give an opinion concerning the cause of the accident, since he had investigated 600 accidents).{/footnote}  A police officer is not qualified to estimate a vehicle’s speed based on vehicle damage and skid marks, however.{footnote}Deaver v. Hickox, 224 N.E.2d 468 (Ill. 1967).{/footnote}  See also ACCIDENT RECONSTRUCTION.

Legal Experts
A prosecutor cannot testify as to whether material meets the legal standard for pornography.{footnote}State v. Watson, 414 P.2d 337 (Or. 1966).{/footnote}  See also PORNOGRAPHY.  Courts have come to different conclusions as to whether a law professor specializing in evidence law may testify as to the reliability of handwriting analysis.{footnote}Compare United States v. Velasquez, 64 F.3d 844, 852 (3d Cir. 1995) (refusing to admit testimony was abuse of discretion), with United States v. Paul, 175 F.3d 906, 912 (11th Cir.1999) (refusing to admit testimony was not abuse of discretion).{/footnote}  Attorneys may testify as expert witnesses as to legal concepts.{footnote}Universal Athletic Sales Co. v. American Gym, Rec. & Athletic Equip. Corp., 546 F.2d 530 (3d Cir. 1976), cert. denied, 430 U.S. 984 (1977) (no abuse of discretion in allowing law firm associate representing several defendants to testify as an expert witness in patent infringement action);United States v. Van Dyke, 14 F.3d 415 (8th Cir. 1994) (error to prohibit attorney from testifying as an expert about federal regulation which she authored and regularly dealt with in practice); McDonnell Douglas Corp. v. Islamic Republic of Iran, 591 F. Supp. 293 (E.D. Mo. 1983), aff’d, 758 F.2d 341 (8th Cir.), cert. denied, 474 U.S. 948 (1985) (an attorney who lived in Iran and who was actively involved in the legal community there could give an opinion concerning the state of the legal system in that country).
But see Harbor Ins. Co. v. Continental Bank Corp., 922 F.2d 357 (7th Cir. 1990) (reversible error to permit plaintiff’s lawyer to testify to the meaning of a crucial term in defendant’s charter); United States v. French, 12 F.3d 114 (8th Cir. 1993) (attorney could not testify as expert that witnesses who sign plea agreements are more likely to falsely incriminate defendants to get sentence reductions).
{/footnote}  An attorney’s ability to testify as a witness on behalf of his client, however, will generally be limited by rules of professional conduct.{footnote}Rule 3.7, A.B.A. Model Rules of Professional Conduct.{/footnote}

Drug users.  Drug users and dealers are qualified to identify a substance as having been a specific narcotic or other drug.{footnote}United States v. Johnson, 575 F.2d 1347 (5th Cir. 1978) (Columbian marijuana); United States v. Atkins, 473 F.2d 308 (8th Cir. 1973) (heroin).
State v. Johnson, 196 N.W.2d 717 (Wis. 1972) (LSD).{/footnote}

Burglars.  An experienced burglar may testify as an expert witness as to the tools used in burglary.{footnote}State v. Briner, 255 N.W.2d 422 (Neb. 1977).{/footnote}

Reliability of Principles or Methods

In Daubert v. Merrell Dow Pharmaceuticals, the Supreme Court held that trial judges must assume the role of gatekeepers in determining the reliability of expert scientific testimony.{footnote}Gruca v. Alpha Therapeutic Corp., 51 F.3d 638 (7th Cir. 1995); Hose v. Chicago Northwestern Transp. Co., 70 F.3d 968 (8th Cir. 1995).{/footnote}  The Court identified five factors courts may consider in determining whether an expert’s methods or discipline are sufficiently reliabe:  (1) whether the offered technique is capable of and has been tested;{footnote}See also Deimer v. Cincinnati Sub-Zero Prod., Inc., 58 F.3d 341, 345 (7th Cir. 1995.
            See also State v. York, 564 A.2d 389, 391 (Me. 1989) (excluding scientific testimony not supported by experimental research); Nelson v. Trinity Medical Ctr., 419 N.W.2d 886, 892 (N.D. 1988) (same); State v. Smith, 362 N.E.2d 1239 (Ohio Ct. App. 1976) (modified version of gunshot residue test held inadmissible where modification not subjected to testing).{/footnote} (2) whether the technique has been subjected to peer review and publication;{footnote}Sorenson By and Through Dunbar v. Shaklee Corp., 31 F.3d 638 (8th Cir. 1994) (excluding testimony as to chemical residue on alfalfa tablets causing mental retardation of children).
{/footnote} (3) the rate of possible error; (4) whether there are "standards controlling the technique’s operation"; and (5) the degree of acceptance by the scientific community.{footnote}Daubert, 509 U.S. at ___, 113 S. Ct. at ___, 125 L. Ed. 2d at ___; Sorenson By and Through Dunbar v. Shaklee Corp., 31 F.3d 638 (8th Cir. 1994)(excluding testimony as to chemical residue on alfalfa tablets causing mental retardation of children).
Followed in M.G. Bancorporation, Inc. v. Le Beau, 737 A.2d 513, 521 (Del. 1999); Mitchell v. Commonwealth,908 S.W.2d 100 (Ky. 1995), overruled on other grounds, Fugate v. Commonwealth, 993 S.W.2d 931 (Ky. 1999); Commonwealth v. Lanigan, 419 Mass. 15, 641 N.E.2d 1342 (1994); State v. Goode, 341 N.C. 513, 461 S.E.2d 631, 639 (1995).{/footnote}  Courts need not always consider all of these factors, since not all will be applicable in a particular case.{footnote}Kumho Tire Co. v. Carmichael, 526 U.S. 137, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999) (upholding district court’s exclusion of expert testimony by tire failure analyst).{/footnote}  In Kumho Tire Co., Ltd. v. Carmichael, the Supreme Court extended Daubert to non-scientific types of expert testimony.{footnote}526 U.S. 137, 141, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999) (upholding district court’s exclusion of expert testimony by tire failure analyst).
Followed in M.G. Bancorporation, Inc. v. Le Beau, 737 A.2d 513, 522 (Del. 1999); Goodyear Tire and Rubber Company v. Thompson, 11 S.W.3d 575 (Ky. 2000); Canavan’s Case, 432 Mass. 304, 733 N.E.2d 1042 (2000).
But see State v. Cline (1996), 275 Mont. 46, 55, 909 P.2d 1171, 1177 (1996) (limiting Daubert approach to “novel” scientific evidence).{/footnote}  In response to Daubert and Kumho, FRE 702 was amended in 2000. 

Where qualified expert testimony in a certain area has been accepted as reliable by an appellate court, a trial court may take judicial notice of the reliability of the method or technique, and need not conduct a Daubert hearing.{footnote}United States v. Martinez, 3 F.3d 1191 (8th Cir. 1993).
Johnson v. Commonwealth, 12 S.W.3d 258 (Ky. 1999) (hair analysis); Commonwealth v. Frangipane, 433 Mass. 527, 538, 744 N.E.2d 25 (2001).{/footnote}

Expert’s Application of Methods to Facts of the Case

In General Electric Co. v. Joiner, the Court noted that trial courts could scrutinize the reliability of a proffered expert’s reasoning process as well as his or her methodology.   “[N]othing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that is connected to existing data only by the ipse dixit of the expert.” {footnote}522 U.S. 136, 146, 118 S.Ct. 512, 519, 139 L.Ed.2d 508, ___ (1997).{/footnote}  Subsequently, the Court has stated that a trial court should satisfy itself that the expert “employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.”{footnote}Kumho Tire Co. v. Carmichael, 526 U.S. 137, 152, 119 S.Ct. 1167, 1176, 143 L.Ed.2d 238 (1999).{/footnote}

3.  Form of Testimony

While opinion testimony is generally inadmissible (see OPINIONS), expert testimony is an exception to the rule.  Experts may give opinions on an ultimate issue to be determined by the trier of fact.{footnote}FRE 704; U.S. v. Crisp, 324 F.3d 261, 271 (4th Cir. 2003) (handwriting expert could testify as to his opinion, not simply the similarities in the writings).

State v. Clifford, 328 Mont. 300, 121 P.3d 489 (Mont. 2005) (handwriting expert’s testimony as to authorship).{/footnote}  Courts may prevent experts from offering opinions on the ultimate legal or factual conclusions to be determined in a case on the ground that the opinion would not be “helpful” to the trier of fact.{footnote}Patterson v. McLean Credit Union, 805 F.2d 1143 (4th Cir. 1986), vacated in part on other grounds, 109 S. Ct. 2363 (1989) (in employment discrimination action, expert testimony that plaintiff was better qualified than person who was promoted was excluded as not helpful to the jury in assessing the relative qualifications of the two people); Mercado v. Austin Police Dep’t, 754 F.2d 1266 (5th Cir. 1985) (no error in the exclusion of an expert’s opinion that discrimination was an element of a personnel decision on the ground that it would not be helpful in an employment discrimination case); McGowan v. Cooper Indus., 863 F.2d 1266 (6th Cir. 1988) (expert in product liability action could testify to industry standards but could not testify that defendant manufacturer was negligent; jury could understand those standards and needed no help to decide whether defendant owed a duty and breached it); Zimmer v. Miller Trucking Co., 743 F.2d 601 (8th Cir. 1984) (no error in the exclusion of police officer’s opinion as to the cause of auto accident where the trial court found that jurors were as able as officer to understand whether or not it was reasonable for a truck to park on the side of the road).
But see Dunn v. HOVIC, 1 F.3d 1362, modified on other grounds, 13 F.3d 58 (3d Cir.), cert. denied, 114 S. Ct. 650 (1993) (no error in allowing the plaintiff’s expert to testify, on the basis of his review of the asbestos manufacturer’s corporate records, that the manufacturer knew of the risks of asbestos); United States v. Lipscomb, 14 F.3d 1236 (7th Cir. 1994) (no error in allowing law enforcement officer to opine that cocaine found on the defendant was for distribution rather than personal use).
See also Note, The Admissibility of Expert Witness Testimony: Time to Take the Final Leap?, 42 U. Miami L. Rev. 831 (1988) (discussing expert testimony embodying conclusions of law); Note, Expert Legal Testimony, 97 Harv. L. Rev. 797 (1984).{/footnote}  See also CREDIBILITY–Opinion Testimony; CUSTOM AND PRACTICE–Proving Custom and Practice; OPINIONS–Ultimate Issues and Legal Opinions; SOCIOLOGICAL EVIDENCE.

4.  Reasonable Basis for Opinion Required

An expert’s opinion will only be admitted where the opinion is based on matters reasonably relied on by experts in the particular field in rendering opinions on the same subject.{footnote}FRE 702, 703; United States v. Watson, 587 F.2d 365 (7th Cir. 1978);
Wilson v. Clark, 84 Ill. 2d 186, 417 N.E.2d 1322 (1981), cert. denied, 454 U.S. 836 (following FRE 703 and FRE 705); Wasserman v. Fifth & Reed Hospital, 660 A.2d 600, 607 (Pa. Super. 1995).{/footnote}  Traditionally, experts have been required to either explain the facts forming the basis for their opinion before stating it, or else render an opinion based on a hypothetical question which includes each of these facts.{footnote}Ingram v. McCruston, 134 S.E. 2d 705 (N.C. 1964).{/footnote}  Some courts also require that the expert express reasonable certainty as to his or her opinion.{footnote}Rohrbough v. Wyeth Labs., 916 F.2d 970 (4th Cir. 1990) (“reasonable probability”); Mayhew v. Bell S.S. Co., 917 F.2d 961 (6th Cir. 1990) (expert testimony was properly excluded because it did not articulate more than a mere possibility of a causal relationship between the defendant’s negligence and the plaintiff’s injury); People of Guam v. Reyes, 879 F.2d 646 (9th Cir. 1989) (expert’s testimony that "it [was] safe to assume" a bruise "probably" was properly admitted because the record clearly showed he based his testimony on the substantive equivalent of “reasonable certainty”).
State v. Mitchell, 390 A.2d 495 (Me. 1978) ("high probability").
But see DaSilva v. American Brands, Inc., 845 F.2d 356 (1st Cir. 1988) (physician permitted to testify as to cause of death even though he used phrases like "his belief" or "could have" in describing his opinion); Schulz v. Celotex Corp., 942 F.2d 204 (3d Cir. 1991) (attending physician in an asbestos product liability action did not have to couch his diagnosis of causation in terms of reasonable medical certainty); United States v. Cyphers, 553 F.2d 1064 (7th Cir.), cert. denied, 434 U.S. 843 (1977) (expert testimony that hairs recovered from articles used by the robbers were "microscopically like" hair samples taken from defendants was properly admitted; lack of certainty goes to the weight of the evidence, not to its admissibility).{/footnote}

Under the federal rules, on the other hand, the expert witness need not disclose the facts on which his or her opinion is based during the direct examination unless the court so orders.{footnote}Vermont Food Industries, Inc. v. Ralston Purina Co., 514 F.2d 456 (2d Cir. 1975); Bezanson v. Fleet Bank-NH, 29 F.3d 16 (1st Cir. 1994) (experts are allowed to tetify to their bare conclusions).
But See Ambrosini v. Labarraque, 966 F.2d 1464 (D.C. Cir. 1992) (trial court erred in granting summary judgment because affidavits from experts who said drug caused birth defects did not disclose data supporting their opinions).{/footnote}  The effect of this rule is to place the burden on the cross-examiner to uncover facts which the expert failed to take into account. 

4(a).  Personal Knowledge

Experts may of course base their opinions or observations they themselves have made.{footnote}Jones v. Otis Elevator Co., 861 F.2d 655 (11th Cir. 1988) (expert with thirty years of experience as an elevator engineer could give an opinion as to the cause of an accident, even though his opinion was based on two elevator inspections that occurred more than two years after the accident).
{/footnote}

4(b).  Hearsay

Expert opinions may be based on the hearsay statements of others as to the underlying facts.  So long as the information relied upon is of a type reasonably relied upon by experts in the field, the information need not be independently admissible.{footnote}FRE 702-703; International Adhesive Coating Co. v. Bolton Emerson Int’l, Inc., 851 F.2d 540 (1st Cir. 1988); Lewis v. Rego, Co., 757 F.2d 66 (3rd Cir. 1985), Peteet v. Dow Chemical Co., 868 F.2d 1428 (5th Cir. 1989); Mannino v. International Mfg. Co., 650 F.2d 846 (6th Cir. 1981); United States v. Lundy, 809 F.2d 392 (7th Cir. 1987); United States v. Mills, 434 F.2d 266 (8th Cir. 1970), cert. denied, 401 U.S. 925 (1971); United States v. Sims, 514 F.2d 147 (9th Cir. 1975); United States v. Farley, 992 F.2d 1122 (10th Cir. 1993).
State v. Rupp, 586 P.2d 1302 (Ariz. 1978); Melecosky v. McCarthy Brothers Co., 115 Ill. 2d 209, 503 N.E.2d 355 (1986) (hearsay statements of patient properly relied on); Farrell v. Plasti-Drum Corp., 512 N.E.2d 1325 (Ill. App. Ct. 1987) (accountant properly relied on working papers of another accountant not introduced into evidence.); Ohio v. Garland, 1996 Ohio App. LEXIS 5660 (Ohio App. Ct. 1996).{/footnote} A hearsay basis, though substantively inadmissible, may properly provide a basis for an admissible opinion.{footnote}Lewis v. Rego, Co., 757 F.2d 66 (3rd Cir. 1985); Mannino v. International Mfg. Co., 650 F.2d 846 (6th Cir. 1981); United States v. Lundy, 809 F.2d 392 (7th Cir. 1987); United States v. Farley, 992 F.2d 1122 (10th Cir. 1993.
Smith v. State, 285 N.E.2d 275 (Ind. 1972), cert. denied, 409 U.S. 129 (1973); Md. Rule 5-703; Beahm v. Shortall, 368 A.2d 1005, 1009 (Md. 1977); Primavera v. Celotex Corp., 608 A.2d 515, 518-19 (Pa. Super. Ct. 1992), appeal denied, 622 A.2d 1374 (Pa. 1993);
{/footnote} The expert’s opinion must not be based entirely on hearsay reports, however.{footnote}United States v. Rollins, 862 F.2d 1282 (7th Cir. 1988); United States v. Lawson, 653 F.2d 299 (7th Cir. 1981); United States v. Kail, 804 F.2d 441 (8th Cir. 1986).
{/footnote}

Some courts require that a limiting instruction be given to the jury which explains that the hearsay evidence is only to be considered as a basis for the expert’s opinion and not as substantive evidence.{footnote}Engebretsen v. Fairchild Aircraft Corp., 21 F.3d 721 (6th Cir. 1994); Paddack v. Dave Christensen, Inc., 745 F.2d 1254 (9th Cir. 1984).
{/footnote} Some courts require that the reliability of the specific information in questions be given special scrutiny.{footnote}Zenith Radio Corp. v. Matsushita Electric Industrial Co., 505 F. Supp. 1190 (E.D. Pa. 1980). 
Contra Weinstein § 703[03], pp. 703-718 (Supp. 1982).
See also University of R.I. v. A.W. Chesterton Co., 2 F.3d 1200, 1218 (1st Cir. 1993) (“Rule 703 requires the trial court to give “careful consideration’ to any inadmissible facts upon which the expert will rely, in order to determine whether reliance is ‘reasonable'”); In re James Wilson Assocs., 965 F.2d 160, 173 (7th Cir. 1992) (under FRE 703, “the judge must make sure that the expert isn’t being used as a vehicle for circumventing the rules of evidence”).{/footnote}  Courts may also admit the expert’s opinion, but prohibit the expert from revealing to the jury the inadmissible factual basis, if the court determines that the risk of prejudice substantially outweighs its probative value.{footnote}Engebretsen v. Fairchild Aircraft Corp., 21 F.3d 721, 729 (6th Cir. 1994); Nachtsheim v. Beech Aircraft Corp., 847 F.2d 1261, 1270-71 (7th Cir. 1988).{/footnote} 

One expert’s opinion may be based in part on consultation with one or more other experts as well,{footnote}Lewis v. Rego, Co., 757 F.2d 66 (3rd Cir. 1985), United States v. Golden, 532 F.2d 1244 (9th Cir. 1976); But see Hatfield v. Sandoz-Wander, 124 Ill. App. 3d 780, 464 N.E.2d 1105 (1st Dis. 1984) (physician unreasonably relied on opinions of other, unqualified physicians).{/footnote} so long as this is customary in the field.{footnote}Jenkins v. United States, 304 F.2d 637 (D.C. Cir. 1962).{/footnote}

4(c).  Evidence at Trial

An expert may attend at trial and generally may render an opinion based on evidence admitted.{footnote}Ballin Drugs v. Dept. of Registration and Education, 166 Ill. App. 3d 520, 519 N.E.2d 1151 (1st Dist. 1988) (expert could base opinion on listening to another expert testify); United States v. Yost, 24 F.3d 99 (10th Cir. 1994) (arson expert‘s testimony regarding the possibility of a staged burglarly prior to fire was not improper where there was ample evidence in record upon which his opinion could have been based).{/footnote} Experts may not, however, render an opinion based on evidence admitted at trial if the evidence was conflicting,{footnote}Kale v. Douthitt, 274 F.2d 476 (4th Cir. 1960).{/footnote} or if the opinion which is based on his assessment of the credibility of particular witnesses.{footnote}United States v. Scop, 846 F.2d 135, 142-43 (2d Cir. 1988) (in dicta, stating, “[W]itness A may not offer an opinion as to relevant facts based on A’s assessment of the trustworthiness or accuracy of witness B where B’s credibility is an issue to be determined by the trier of fact.”); United States v. Whitted, 11 F.3d 782 (8th Cir. 1993); (physician’s opinion that the victim had suffered repeated child sexual abuse was improperly admitted; the physical evidence and examination were inconclusive, so the conclusion had to be based on the doctor’s belief that the victim’s claims of abuse were true); United States v. Toledo, 985 F.2d 1462 (10th Cir.), cert. denied, 114 S. Ct. 218 (1993) (it was a close question, but not plain error, to admit a psychiatrist’s testimony as to whether the victim’s story might be delusional in a prosecution for kidnapping a fourteen-year-old mentally retarded and mentally ill girl).
But see In re Air Disaster at Lockerbie Scotland, 37 F.3d 804, 828 (2d Cir. 1994) (describing above-quoted language in Scop as dicta); United States v. Plenty Arrows, 946 F.2d 62 (8th Cir. 1991) (physician’s testimony that victim’s denial of sexual abuse was consistent with the behavior of a sexually abused child was not improper); United States v. Antone, 981 F.2d 1059 (9th Cir. 1992); (expert testimony regarding the general behavioral characteristics exhibited by victims of child sexual abuse was not improper where the expert did not testify regarding the victims’ credibility); Margaret A. Berger, The Second Circuit Review — 1987-1988 Term: Evidence: United States v. Scop: The Common-Law Approach To An Expert’s Opinion About A Witness’s Credibility Still Does Not Work, 55 Brooklyn L. Rev. 559 (1989) (criticizing Scop).
CHECK 3 J. Weinstein & M. Berger, Weinstein’s Evidence ¶ 705[01], at 705-6 to -10 (1987).

{/footnote}

4(d).  Hypothetical Facts

An expert may also render an opinion based on hypothetical facts which correlate with the evidence admitted at trial, or to be introduced later.{footnote}FRE 705.{/footnote}  Many states require that all expert testimony be presented this way.{footnote}Check Cal. sec. 802, comment; 56 A.L.R. 3d 300.{/footnote}  Where the expert is testifying based on hypothetical facts which counsel expects to be substantiated later in the trial, some courts will first require an offer of proof.{footnote}Takoma Park Bank v. Abbot, 19 A.2d 169 (Md.).{/footnote}

Under the federal rules, hypothetical questions need include only those facts that lead the expert to his or her opinion.  Facts which are inconsistent with the opinion may be left out of the hypothetical, and used by the opponent on cross-examination.{footnote}FRE 705; Vermont Food Industries, Inc. v. Ralston Purina Co., 514 F.2d 456 (2d Cir. 1975).{/footnote}  Some states, however, require that all material facts be included in the hypothetical, whether or not they lend support to the expert’s opinion.{footnote}Ames & Webb, Inc. v. Commercial Laundry Co., 33 S.E. 2d 547 (Va. 1963).
United States v. Wilson, 798 F.2d 509 (1st Cir. 1986) (the trial court properly excluded an expert from testifying, where the expert was expected to answer hypothetical questions unsupported by facts proved by the defense); Theriot v. Bay Drilling Corp., 783 F.2d 527 (5th Cir. 1986) (hypothetical questions posed to an expert without personal knowledge must assume a fair, undistorted collection of facts).{/footnote}

On cross-examination, the probative value of an expert’s opinion may be attacked by changing or adding to the facts to be assumed by the expert, and asking the expert if the new or different facts would change his or her opinion.

4(e).  Publications

Expert testimony may be based in part on information contained in learned treatises or scholarly journals so long as they are authoritative.{footnote}Check United States v. Golden, 532 F.2d 1244 (9th Cir. 1976); Mielke v. Condell Memorial Hospital, 463 N.E.2d 216 (Ill. App. Ct. 1984).{/footnote}  In some courts, the information itself may be read and admitted as substantive evidence either on direct or on cross-examination.  See LEARNED TREATISES.

4(f). Experiments, Reconstructions, and Demonstrations

An expert opinion may be based on experiments,{footnote}Young v. Illinois Cent. Gulf R.R., 618 F.2d 332 (5th Cir. 1980) (trial court erred in excluding expert testimony based on an experiment which tended to show how dangerous a railroad crossing was).
{/footnote} reconstructions,{footnote}Bauman v. Volkswagenwerk A.G., 621 F.2d 230 (6th Cir. 1980) (expert testimony based on simulated reproductions of an accident was proper).
{/footnote} and demonstrations{footnote}Kehm v. Procter & Gamble Mfg. Co., 724 F.2d 613 (8th Cir. 1983) (plaintiffs’ expert was properly permitted to perform an in-court demonstration of how a component in the manufacturer’s tampons could interact with an enzyme found in the vagina to create hospitable conditions for the growth of a bacteria that produces TSS toxin).
{/footnote} conducted at trial or outside of the presence of the trier of fact.  The tests must be performed, however, under conditions sufficiently similar to the actual conditions surrounding the subject of the expert’s opinion.{footnote}Jackson v. Fletcher, 647 F.2d 1020 (10th Cir. 1981) (reversible error to admit expert testimony based upon experiments conducted by the defendants’ experts when the circumstances of the experiments were different from those of the accident); Barnes v. General Motors Corp., 547 F.2d 275 (5th Cir. 1977) (error to allow plaintiff’s expert to testify about experiment he conducted on another automobile under significantly different circumstances from those admittedly existing at the time of accident at issue).

{/footnote}

Other Grounds for Exclusion

Expert testimony which is merely cumulative may properly be excluded,{footnote}In re Marriage of Weinstein, 470 N.E.2d 551 (Ill. App. Ct. 1984) (expert testimony as to earning capacity of husband).
But see Johnson v. United States, 780 F.2d 902 (11th Cir. 1986) (judge in bench trial erred by excluding plaintiffs’ third expert witness as cumulative where expert’s credentials were better than other two witness’ and analysis was different).{/footnote} although a party may buttress one expert’s testimony given in its case in chief with similar testimony by another expert in its rebuttal case, where the other side has presented opposing expert testimony.{footnote}Kupcikevicius v. Fitzgibbons, 354 N.E.2d 434 (Ill. App. Ct. 1976).{/footnote}  Relevant expert testimony may also be excluded as substantially more prejudicial than probative.{footnote}Buscaglia v. United States, 25 F.3d 530 (7th Cir. 1994); Rogers v. Raymark Industries, Inc. 922 F.2d 1426 (9th Cir. 1991) (expert in marine asbestos insulation techniques).

But see In re Paoli R.R. Yard PCB Litigation, 35 F.3d 717 (3d Cir. 1994), cert. denied General Elec. Co. v. Ingram, 115 S.Ct. 1253, 131 L.Ed.2d 134 (expert testimony as in toxic tort case as to causation improperly excluded); Petruzzi’s IGA Supermarkets, Inc. v. Darling-Delaware Co., 998 F.2d 1224 (3d Cir.), cert. denied, 114 S. Ct. 554 (1993) (In finding that the trial court erred by excluding expert testimony under FRE 403, the court reasoned that "if expert testimony survives the rigors of Rules 702 and 703, then Rule 403 becomes an unlikely basis for exclusion, especially in the pretrial setting").{/footnote}

5.  Impeachment

Expert witnesses may be impeached on all the same grounds as ordinary witnesses. See IMPEACHMENT.  In addition, experts may be impeached by attacking their qualifications (see Qualifying an Expert, above) and by several other means.  It is improper to impeach an expert witness by eliciting from another expert witness what he thinks of that expert.{footnote}Carver v. Orange County, 444 So. 2d 452 (Fla. App. 1983).{/footnote}

8

5(a).  Bias

Parties may impeach expert witnesses by cross-examining them as to their being paid to testify,{footnote}Cal. Evid. Code § 722.{/footnote} the income they receive from testifying generally,{footnote}Collins v. Wayne Corp., 621 F.2d 777 (5th Cir. 1980) (cross-examination of an expert about fees earned in prior cases was not improper because such evidence might tend to establish bias).
{/footnote} or the comparative frequency with which they testify for plaintiffs or defendants.{footnote}United States v. Edwardo-Franco, 885 F.2d 1002 (2d Cir. 1989); Gwathmey v. United States, 215 F.2d 148 (5th Cir. 1954) (government’s expert impeached by fact that he generally testified for government in condemnation actions).

See also Trower v. Jones, 121 Ill. 2d 211, 520 N.E.2d 297 (1988).{/footnote}

5(b).  Contrary Opinions

An expert may be impeached by showing that he or she rendered a contrary opinion previously in the same case, but contrary opinions stated in other cases are held by most courts to be inadmissible.

5(c).  Authoritative Publications

Where an expert has testified that he or she relied on an authoritative publication, the expert may of course be impeached by showing that the publication does not support the expert’s opinion.  There is a split of authorities, however, as to whether one may impeach an expert with other authoritative publications upon which the expert does not claim to rely. Under the federal rules and in most states such cross-examination is permitted.{footnote}FRE 803(18).{/footnote}   The publication may be established as authoritative either through the admission of the expert being cross-examined, through the testimony of another expert, or through the court taking judicial notice of the publication’s authoritativeness.{footnote}FRE 803(18).
See also Carroll v. Morgan, 17 F.3d 787 (5th Cir. 1994) (no abuse of discretion in permitting the defendant’s expert to testify regarding the cause of death, despite his reluctance to accept as authoritative in their entirety certain medical textbooks and journals mentioned by the plaintiff and to accept one particular source as the exclusive authority on endocarditis).{/footnote}  The publication is generally admitted only for the purpose of impeachment, however, and not as substantive evidence.{footnote}Ruth v. Fenchel, 121 A.2d 373 (N.J. 1956).{/footnote}  Some states do not permit this form of cross-examination of experts at all.{footnote}Cal. § 721(b).{/footnote}

6.  Admissions

Statements by experts are admissible against the party that hired them,{footnote}FRE 801(d)(2)(C); Collins v. Wayne Corp.; 621 F.2d 777, 782 (5th Cir. 1980).{/footnote} but only where the statements were made by an expert who was within the control of the party opponent.{footnote}Kirk v. Raymark Industries, Inc., 61 F.3d 147 (3d Cir. 1995), cert. denied, 116 S.Ct. 1015, 134 L.Ed.2d 95 (expert’s testimony in prior unrelated litigation held inadmissible); Sanford v. Johns-Manville Sales Corp., 923 F.2d 1142 (5th Cir. 1991) (looking to Texas law in deciding that examining physician was not an agent of asbestos manufacturers).{/footnote}  See ADMISSIONS–Vicarious Admissions: Agents and Employees.

Court-Appointed Experts

Although expert witnesses are typically brought into litigation by the parties, a court can appoint an expert witness or a panel of expert witnesses on its own motion or on the motion of any party.{footnote}FRE 706; Computer Assoc. Intl Inc. v. Altai, Inc., 982 F.2d 693 (2nd Cir. 1992) (expert’s opinion was insturmental in dismantling the intricacies of computer science in copyright infringement action).
{/footnote}  Whether an expert witness should be appointed is a matter within the trial court’s discretion.{footnote}United States v. Larkin, 978 F.2d 964 (7th Cir. 1992); Jackson v. Ylst, 921 F.2d 882 (9th Cir., 1990).
{/footnote}

7.  Administrative Proceedings

Administrative agencies have the discretion to admit expert testimony but the agency may not exercise its discretion arbitrarily.{footnote}Alabama Assoc. of Ins. Agents v. Bd. of Governors of the Federal Reserve System, 533 F.2d 224, 254 (5th Cir. 1976), vac’d in pt., 558 F.2d 729 (5th Cir. 1977); Pagel, Inc. v. SEC, 803 F.2d 942, 947 (8th Cir. 1986); Yaffe Iron & Metal Co. v. United States EPA, 774 F.2d 1008, 1016 (10th Cir. 1985).{/footnote}

8.  Appellate Review

The trial judge has broad discretion in admitting or excluding expert testimony.  See §§ 1, 2, infra.  As with other issues of admissibility, a lower court’s decision as to the admissibility of expert testimony will only be reviewed for an abuse of discretion.{footnote}General Electric v. Joiner, 522 U.S. 136, 146, 118 S.Ct. 512, 519, 139 L.Ed.2d 508, ___ (1997).
Johnson v. Lynch, 574 A.2d 934, 939 (N.H. 1990); Howerton v. Arai Helmet, 158 N.C. App. 316, 333, 581 S.E.2d 816, 827 (2003); Ohio v. Garland, 1996 Ohio App. LEXIS 5660 (Ohio App.1996); Billiter v. Melton Truck Lines, Inc., 420 S.E.2d 286, 291 (W. Va. 1992).
{/footnote}  Whether the court applied the correct legal standard is reviewed de novo.{footnote}Deputy v. Lehman Bros., ____F.3d ____ (7th Cir. 2003).{/footnote}  See also APPEAL.

Bibliography

Note, Expert Legal Testimony, 97 Harv. L. Rev. 797 (1984).