|Part of the common law series|
|Types of evidence|
|Hearsay and exceptions|
|Other common law areas|
An expert witness, professional witness or judicial expert is a witness, who by virtue of education, training, skill, or experience, is believed to have expertise and specialised knowledge in a particular subject beyond that of the average person, sufficient that others may officially and legally rely upon the witness's specialized (scientific, technical or other) opinion about an evidence or fact issue within the scope of his expertise, referred to as the expert opinion, as an assistance to the fact-finder. Expert witnesses may also deliver expert evidence about facts from the domain of their expertise. At times, their testimony may be rebutted with a learned treatise, sometimes to the detriment of their reputations.
In Scots Law, Davie v Magistrates of Edinburgh (1953) provides authority that where a witness has particular knowledge or skills in an area being examined by the court, and has been called to court in order to elaborate on that area for the benefit of the court, that witness may give evidence of his opinion on that area.
Role of expert witnesses
Typically, experts are relied on for opinions on severity of injury, degree of sanity, cause of failure in a machine or other device, loss of earnings, care costs, and the like. In an intellectual property case, an expert may be shown two music scores, book texts, or circuit boards and asked to ascertain their degree of similarity. In the majority of cases the expert's personal relation to the defendant is considered irrelevant.
The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties.
The expert has a heavy responsibility, especially in penal trials, and perjury by an expert is a severely punished crime in most countries. The use of expert witnesses is sometimes criticized in the United States because in civil trials, they are often used by both sides to advocate differing positions, and it is left up to a jury to decide which expert witness to believe. Although experts are legally prohibited from expressing their opinion of submitted evidence until after they are hired, sometimes a party can surmise beforehand, because of reputation or prior cases, that the testimony will be favorable regardless of any basis in the submitted data; such experts are commonly disparaged as "hired guns."
Duties of experts
In England and Wales, under the Civil Procedure Rules 1998 (CPR), an expert witness is required to be independent and address his or her expert report to the court. A witness may be jointly instructed by both sides if the parties agree to this, especially in cases where the liability is relatively small.
Under the CPR, expert witnesses may be instructed to produce a joint statement detailing points of agreement and disagreement to assist the court or tribunal. The meeting is held quite independently of instructing lawyers, and often assists in resolution of a case, especially if the experts review and modify their opinions. When this happens, substantial trial costs can be saved when the parties to a dispute agree to a settlement. In most systems, the trial (or the procedure) can be suspended in order to allow the experts to study the case and produce their results. More frequently, meetings of experts occur before trial. Experts charge a professional fee which is paid by the party commissioning the report (both parties for joint instructions) although the report is addressed to the court. The fee must not be contingent on the outcome of the case. Expert witnesses may be subpoenaed (issued with a witness summons), although this is normally a formality to avoid court date clashes.
In the United States, under the Federal Rule of Evidence 702 (FRE), an expert witness must be qualified on the topic of testimony. In determining the qualifications of the expert, the FRE requires the expert have specialized education, training, or practical experience in the subject matter relating to the case. The expert's testimony must be based on facts in evidence, and should offer opinion about the causation or correlation to the evidence in drawing a conclusion.
Experts in the U.S. typically are paid on an hourly basis for their services in investigating the facts, preparing a report, and if necessary, testifying during pre-trial discovery, or at trial. Hourly fees range from approximately $200 to $750 or more per hour, varying primarily by the expert's field of expertise, and the individual expert's qualifications and reputation. In several fields, such as handwriting analysis, where the expert compares signatures to determine the likelihood of a forgery, and medical case reviews by a physician or nurse, in which the expert goes over hospital and medical records to assess the possibility of malpractice, experts often initially charge a flat fixed fee for their initial report. As with the hourly fees discussed previously, the amount of that flat fee varies considerably based on the reviewing expert's field, experience and reputation.  The expert's professional fee, plus his or her related expenses, is generally paid by the party retaining the expert. In some circumstance the party who prevails in the litigation may be entitled to recover the amounts paid its expert from the losing party. In high stakes cases multiple experts, in multiple topics, are often retained by each party. Although it is still relatively rare, the court itself may also retain its own independent expert. In all cases, fees paid to an expert may not be contingent on the outcome of the case.
The earliest known use of an expert witness in English law came in 1782, when a court that was hearing litigation relating to the silting-up of Wells harbour in Norfolk accepted evidence from a leading civil engineer, John Smeaton. This decision by the court to accept Smeaton's evidence is widely cited as the root of modern rules on expert evidence. However, it was still such an unusual feature in court that in 1957 in the Old Bailey, Lord Justice Patrick Devlin could describe the case of suspected serial killer Dr John Bodkin Adams thus: "It is a most curious situation, perhaps unique in these courts, that the act of murder has to be proved by expert evidence."
On the other hand, expert evidence is often the most important component of many civil and criminal cases today. Fingerprint examination, blood analysis and DNA fingerprinting are common kinds of expert evidence heard in serious criminal cases. In civil cases, the work of accident analysis, forensic engineers, and forensic accountants is usually important, the latter to assess damages and costs in long and complex cases. Intellectual property and medical negligence cases are typical examples.
Electronic evidence has also entered the courtroom as critical forensic evidence. Audio and video evidence must be authenticated by both parties in any litigation by a forensic expert who is also an expert witness who assists the court in understanding details about that electronic evidence.
Voice-mail recordings and closed-circuit television systems produce electronic evidence often used in litigation, more so today than in the past. Video recordings of bank robberies and audio recordings of life threats are presented in court rooms by electronic expert witnesses.
In the U.S., a party may hire experts to help them evaluate a given case. For example, a car maker may hire an experienced mechanic to decide if its cars were built to specification. This kind of expert opinion will be protected from discovery by the opposing party. In other words, if the expert finds evidence against their client, the opposite party will not automatically gain access to it. This privilege is similar to the work-product doctrine (not to be confused with attorney–client privilege).
The non-testifying expert can be present at the trial or hearing to aid the attorney in asking questions of other expert witnesses. Unlike a testifying expert, a non-testifying expert can be easily withdrawn from a case. It is also possible to change a non-testifying expert to a testifying expert before the expert disclosure date. 
If the witness needs to testify in court, the privilege is no longer protected. The expert witness's identity and nearly all documents used to prepare the testimony will become discoverable. Usually an experienced lawyer will advise the expert not to take notes on documents because all of the notes will be available to the other party.
An expert testifying in a United States federal court must satisfy the requirements of Fed. R. Evid. 702. Generally, under Rule 702, an expert is a person with "scientific, technical, or other specialized knowledge" who can "assist the trier of fact," which is typically a jury. A witness who is being offered as an expert must first establish his or her competency in the relevant field through an examination of his or her credentials. The opposing attorney is permitted to conduct a voir dire of the witness in order to challenge that witness’ qualifications. If qualified by the court, then the expert may testify "in the form of an opinion or otherwise" so long as: "(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."
Although experts can testify in any case in which their expertise is relevant, criminal cases are more likely to use forensic scientists or forensic psychologists, whereas civil cases, such as personal injury, may use forensic engineers, forensic accountants, employment consultants or care experts. Senior physicians – UK, Ireland, and Commonwealth consultants, U.S. attending physicians – are frequently used in both the civil and criminal courts.
The Federal Court of Australia has issued guidelines for experts appearing in Australian courts. This covers the format of the expert's written testimony as well as their behaviour in court. Similar procedures apply in non-court forums, such as the Australian Human Rights and Equal Opportunity Commission.
Types of expert witness
The educating witness teaches the fact-finder (jury or, in a bench trial, judge) about the underlying scientific theory and instrument implementing theory. This witness is an expert witness, called to elicit opinions that a theory is valid and the instruments involved are reliable. The witness must be accredited as an expert witness, which may require academic qualifications or specific training.
Called after teaching witness leaves stand. Usually the laboratory technician who personally conducted the test. Witness will describe both the test and the results. When describing test, will venture opinions that proper test procedures were used and that equipment was in good working order.
The Frye test, coming from the case Frye v. United States (1923), said that admissible scientific evidence must be a result of a theory that had "general acceptance" in scientific community. This test results in uniform decisions regarding admissibility. In particular, the judges in Frye ruled that:
- Just when a scientific principle or discovery crosses the line between experimental and demonstrable stages is difficult to define. Somewhere in this twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from a well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs.
This test has been criticized as misunderstanding the scientific process and being based on the assumption that a jury is unable to evaluate scientific testimony. The goals of the test were to avoid evidence from overly questionable or controversial scientific theories to be used; it was used to exclude lie-detector results employed by the defense in the original case.
- That the theory is testable (has it been tested?)
- That the theory has been peer reviewed, (Peer reviewing usually reduces the chances of error in the theory)
- The reliability and error rate (100% reliability and zero error are not required, but the rates should be considered by the trial judge)
- The extent of general acceptance by the scientific community
- Ambush defence
- Daubert Standard and Daubert v. Merrell Dow Pharmaceuticals
- Death of an Expert Witness - a novel
- Employment consultant
- Expert shopping
- Forensic accountant
- Forensic economics
- Forensic engineering
- Forensic science
- Forensic psychology
- Forensic video analysis
- Frye Standard of evidence
- Gibson's law
- In limine
- Jones v Kaney — English caselaw abolishing witness immunity from civil action for negligence
- Kumho Tire Co. v. Carmichael
- Questioned document examination
- R. v. Mohan — Canadian caselaw establishing qualifications for expert witnesses
- Ultimate issue
- Vehicular accident reconstruction
- Federal Rules of Evidence - 2011 | Federal Evidence Review
- Black's Law Dictionary, articles "Evidence", "Expert", "Witness"
- Davie v Magistrates of Edinburgh 1953 SC 34
- Snow, J.N., & Weed, R. (1997). Mental health forensic issues in Georgia: The role of the expert witness. Georgia Journal of Professional Counselors, 53-65.
- Snow, J.N. & Weed, R. (1996). Forensic issues in mental health: The role of the expert witness. Journal of Legal Nurse Consulting, 7(4), 2-13.
- Talve, Michael. "What Is an Expert Witness?". The Expert Institute. Retrieved 14 January 2013.
- Goldsholle, Gerry H. "ExpertPages Expert Witness Fees & Practices Survey". ExpertPages. Retrieved 27 April 2015.
- Cullen, Pamela V., "A Stranger in Blood: The Case Files on Dr John Bodkin Adams", London, Elliott & Thompson, 2006, ISBN 1-904027-19-9
- Eri J.D., Christopher. "What are the differences between an expert witness and a consultant non testifying expert". Forensis Group. Retrieved 19 November 2013.
- Guidelines for Expert Witnesses in Proceedings in the Federal Court of Australia, Practice Direction, (Federal Court of Australia, 2007)
- The accidental expert witness, Tom Worthington, Information Age (IDG, 2005)
- Berger, Margaret A. (2011). "The Admissibility of Expert Testimony". In Federal Judicial Center; National Research Council. Reference Manual on Scientific Evidence (3rd. ed.). Washington, DC: National Academies Press. pp. 11–36. ISBN 978-0-309-21421-6.
- Bronstein, DA, Law for the Expert Witness, CRC Press,2nd Ed (1999).
- Dwyer, D, The Judicial Assessment of Expert Evidence, Cambridge University Press (2008).
- Federal Judicial Center; National Research Council, eds. (2011). Reference Manual on Scientific Evidence (3rd. ed.). Washington, DC: National Academies Press. ISBN 978-0-309-21421-6.
- Jasanoff, Sheila, Science at the Bar: Law, Science, and Technology in America (Cambridge, Mass.: Harvard University Press, 1997).
- Reynolds, MP and King, PSD, The Expert Witness and his Evidence, Blackwell (1992).
- Smith, D, Being an Effective Expert Witness, Thames Publishing (1993).
- Experts & Institutions: Centre for the study of expert knowledge in law and society 
- Expert Testimony in Federal Civil Trials: A Preliminary Analysis (pdf) (Federal Judicial Center, 2000)
- Project on Scientific Knowledge and Public Policy. Daubert-The Most Influential Supreme Court Ruling You've Never Heard Of (pdf)
- Kenton K. Yee, Dueling Experts and Imperfect Verification, 28.4 International Review of Law and Economics, 246-255 (2008)
- Cole, Simon A. "Where the Rubber Meets the Road: Thinking about Expert Evidence as Expert Testimony" (Archive). Villova Law Journal. Villanova University School of Law. Volume 52, Issue 4, Article 4. p. 803-840.