Forensic Evidence

Stephen A Brunette. McGraw-Hill Encyclopedia of Science & Technology. 10th edition, Volume 7, McGraw-Hill, 2007.

Scientific, technical, or other specialized knowledge applied in courts of justice. Courts are finding that some areas of knowledge that have been considered forensic sciences for decades are not based completely on scientific knowledge under contemporary legal and academic standards, but rather are based on technical or other specialized knowledge that must be evaluated under standards not clearly defined. Indeed, some of the most interesting issues facing forensic evidence arise in cases where the defining characteristics of “scientific,” “technical,” and “other” specialized knowledge are at issue.

Federal law in the United States now requires trial courts to assure that forensic evidence is reliable and valid—and not merely generally accepted by others in the witness’s field—before it is admitted in court. The highest courts of several states have also adopted this federal standard.

Historical Context

Several areas of forensic specialization matured in the United States under the “general acceptance” standard announced in Frye v. United States, 293 F 1013 (D.C. Cir., 1923). This standard allowed scientific evidence to be admitted in court if the evidence was generally accepted as reliable by others in the expert’s field. In 1993 in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579, the U.S. Supreme Court rejected the Frye standard as the sole test for admissibility in federal courts, and interpreted the Federal Rules of Evidence to require proof that scientific evidence was valid before it would be found sufficiently reliable to be admitted into evidence. The Court noted that “scientists typically distinguish between ‘validity’ (does the principle support what it purports to show?) and ‘reliability’ (does application of the principle support consistent results?),” but stated that “our reference here is to evidentiary reliability—that is, trustworthiness.… In a case involving scientific evidence, evidentiary reliability will be based upon scientific validity.” In 1999, in Kumho Tire Co. v. Carmichael, 526 U.S. 137, the Supreme Court made it clear that Daubert ‘s requirements of validity and reliability apply not only to scientific evidence but to all types of forensic evidence, and stated that forensic evidence must have “a reliable basis in the knowledge and experience of the witness’s discipline.”

Many types of forensic evidence used in criminal proceedings were developed to meet the practical needs of law enforcement and prosecutorial functions of government, and not specifically to address theoretical or empirical questions. Courts are finding that many areas of forensic specialization are not grounded in science or scientific method, and are just beginning to formulate questions that may allow empirical investigation of the validity and reliability of their fields of knowledge. Areas of forensic knowledge at this stage of development are easy targets for challenges to reliability and validity in litigation.

Areas of forensic specialization that are grounded in science and scientific method are open to challenge on any aspect of theory or practice that may affect the reliability or validity of the evidence. Testimony of a witness or a peer that an established theory or technique has been applied in a manner that is generally accepted in the field is no longer sufficient to allow the evidence to be admitted. The trial court may consider general acceptance in evaluating the reliability and validity of the evidence, but independent proof of reliability and validity is also required.

Validity and Reliability

The shift from “general acceptance” to “validity” and “reliability” is guided primarily by three elements of Federal Rule of Evidence 702 and state counterparts. This rule requires that (1) admissible forensic evidence must consist of “scientific, technical, or other specialized knowledge” (2) that will “assist the trier of fact to understand the evidence or to determine a fact in issue,” and (3) must be provided by a witness who is qualified as an expert by “knowledge, skill, experience, training or education.”

Specialized Knowledge

Daubert defined the “specialized knowledge” element to require “scientific validity” as a basis for “evidentiary reliability.” Applied to nonscientific evidence, Kumho Tire requires forensic evidence to have “a reliable basis in the knowledge and experience of the witness’s discipline.” Both Daubert and Kumho Tire identified four factors that trial courts may use to evaluate the validity and reliability of forensic evidence: (1) whether the knowledge or technique can be (and has been) tested, (2) whether it has been subjected to peer review and publication, (3) the technique’s known or potential error rate, and (4) whether it is generally accepted in the witness’s community. The Supreme Court cautioned in Daubert that this was not a “checklist” and that trial courts could consider anything that addressed the validity and reliability of the evidence. Later, in Kumho Tire, the Supreme Court stated that the trial judge has discretion to decide which, if any, of the four factors to employ in evaluating the reliability and validity of forensic evidence. The range of possible challenges to reliability and validity under these standards is limited only by the creativity of litigants and proponents of areas of specialized knowledge in our increasingly complex society.

Intradisciplinary Knowledge

A forensic expert’s knowledge of the scope and limits of the area of specialized knowledge claimed by the witness is, of course, subject to cross-examination under any or all of the four Daubert factors, and other factors raised by counsel and accepted by the court. A witness should therefore be knowledgeable of authoritative texts, treatises, standards, and training materials that constitute the knowledge base of the witness’s claimed area of expertise. Opposing counsel may produce these materials and ask the witness to identify those deemed authoritative in the witness’s field, and then request the witness to explain any discrepancies between the witness’s testimony or practices and the content of the authoritative treatises. The witness may also be asked to provide a rational, logically and substantively valid explanation of the theories, techniques, standards, customs, and practices of the witness’s field of specialization, and how these were applied to the facts of the case to reach the witness’s opinions or conclusions. The purpose of this type of cross-examination is to identify any acts, omissions, biases, prejudices, or other matters that may affect the validity and reliability of the witness’s evidence, and to provide a basis for counsel to assure that the evidence offered by the witness is within the scope of the witness’s expertise.

Extradisciplinary Knowledge

More importantly, a witness should attempt to be prepared for cross-examination concerning areas of knowledge that may be beyond the witness’s area of specialization but are relevant to the determination of the validity and reliability of the witness’s opinions or conclusions. Daubert ‘s expansion of the judicial inquiry beyond the realm of general acceptance in the witness’s community supports this type of challenge, and opposing counsel may rely on this attribute of Daubert to test the limits of a witness’s knowledge.

For example, the definition of “science” is at issue in an increasing number of cases. A witness from a field of specialized knowledge that is claimed to be a science may be challenged not only concerning the validity and reliability of evidence as viewed from within the witness’s field of knowledge, but also concerning the validity and reliability of the witness’s evidence as viewed from the field of science. Cross-examination may, for example, address how the witness’s evidence accords with definitions of “science” and “scientific method,” or with definitions of particular areas of forensic science or forensic specialization.

Criteria for “science” may be found in the vast literature in this field. For example, nine guideposts that courts and attorneys may use to prepare and challenge scientific evidence have been proposed [B. Black and P. Lee, 1997]: (1) the explanatory power of the evidence to explain, clarify, and predict relationships in general and in the case at hand; (2) logical consistency of the evidence; (3) falsifiability; (4) the variety and severity of testing of hypotheses on which the evidence is based; (5) precision and objectivity of the reasoning and conclusions, rather than testimony consisting of broad generalizations and subjective impressions; (6) post-hypothesis testing, that is, whether the testimony is based on an ad hoc reinterpretation of existing data or on knowledge obtained through successful predictions and observations; (7) consistency of the opinion with accepted theories, that is, general acceptance in the witness’s and related communities of specialization; (8) application and use of the knowledge of the witness by the scientific community outside the context of litigation; and (9) peer review and publication of the knowledge base or, where proprietary information is involved, other internal review.

Though not required under Daubert or Kumho Tire, these and other authoritative criteria for determining whether a field of knowledge is science may be invoked by counsel as a basis for establishing or challenging an area of specialized knowledge that is claimed to be a science. Such knowledge-based inquiries into the validity and reliability of a particular field of specialization are supported (and arguably required) by Daubert ‘s focus on validity and reliability. Challenges to forensic evidence from fields of knowledge outside the witness’s field of knowledge may become the rule, rather than the exception, for cross-examination of witnesses in fields of forensic knowledge that have matured under the protection of general acceptance by only their guilded peers.

Assistance to Trier of Fact

Whether the forensic evidence will assist the trier of fact to understand the evidence or determine a fact in issue has been defined as whether there is a “fit” between the evidence and the issues in the case. Valid, reliable evidence offered by a qualified witness will be inadmissible if it does not assist the trier of fact. The Supreme Court’s example of this element makes the point very well.

“A study of the phases of the moon, for example, may provide valid scientific ‘knowledge’ about whether a certain night was dark, and if darkness is a fact in issue, the knowledge will assist the trier of fact. However (absent creditable grounds supporting such a link), evidence that the moon was full on a certain night will not assist the trier of fact in determining whether an individual was unusually likely to have behaved irrationally on that night. Rule 702’s ‘helpfulness’ standard requires a valid scientific connection to the pertinent inquiry as a precondition to admissibility.”


The threshold inquiry into the witness’s qualifications may address any and all formal education the witness has or has not completed. Education in the area of specialized knowledge in which the witness claims to be proficient will be most relevant. Any publications or other educational materials authored or edited by the witness may be reviewed and addressed on cross-examination, particularly if they make statements inconsistent with any statements made by the witness in the present case. Professional or technical training in the area of specialization, the witness’s performance in that training, and certifications or other credentials related to the areas of specialization may also be addressed.

The witness’s experience in the field of specialized knowledge may also be covered, together with any issues related to the witness’s competent performance in the field. Performance evaluations, disciplinary proceedings, and any other evidence relevant to the witness’s experience may be scrutinized. Testimony of the witness in other court proceedings may also be addressed if it may be used to undermine the reliability or validity of the witness’s opinion in the case at hand.


Though each deposition and court appearance is a unique experience, forensic witnesses may reasonably expect that direct and cross examination will cover at least the key areas addressed above. Direct examination will progress in a manner that allows the witness to favorably state the witness’s education, training, and experience in the witness’s field of specialized knowledge; the facts of the case on which the witness relied in preparing any reports, summaries, or formulating any opinions for the case; any theories, techniques, methods, or procedures applied by the witness in understanding and interpreting the facts; and any conclusions or opinions the witness may have reached as a result of this process. Direct examination is typically a smooth, comfortable exposition of the witness’s qualifications, reasoning, and opinion.

Cross-examination by opposing counsel is typically more challenging. The primary goal of cross-examination is to identify any weaknesses that may undermine the relevance, reliability, and/or validity of the witness’s evidence. Weaknesses will be sought in the witness’s qualifications; knowledge of the scope, limits, validity, and reliability of the witness’s area of specialization; application of the area of specialized knowledge to the facts of the case; and/or relevance of the evidence to the issues in the case. The forensic witness must, therefore, be prepared for intense scrutiny in any of these areas.

The intensity of cross-examination across the range of areas that may affect validity and reliability of forensic evidence has occasionally been interpreted by some forensic witnesses as a personal attack. Personalities aside, the forensic witness may be better prepared to face intense cross-examination by recalling that the forensic expert is, by definition, present to serve the court, and counsel is present to serve the court and a client. Counsel has a duty to assure that evidence presented against the client is valid and reliable; the forensic witness may request assistance from the court if cross-examination interferes with the witness’s ability to serve the court’s fact-finding function.

Court-appointed Experts

Trial courts have the authority to appoint independent experts to consult with the court concerning issues presented in a case, but they rarely use this authority. Daubert ‘s instruction that the trial judge serve as “gate-keeper” may increase the frequency of appointment of experts by courts. The American Association for the Advancement of Science has launched a program to assist federal judges in identifying appropriate court-appointed experts in civil cases.