Evidence — opinions and expert testimony

Wis. Stat. ch. 907
in effect

Wis. Stat. § 907.01 Opinion testimony by lay witnesses

If the witness is not testifying as an expert, the witness’s testimony in the form of opinions or inferences is limited to those opinions or inferences which are all of the following: (1) Rationally based on the perception of the witness. (2) Helpful to a clear understanding of the witness’s testimony or the determination of a fact in issue. (3) Not based on scientific, technical, or other specialized knowledge within the scope of a witness under s. 907.02 (1).

Wis. Stat. § 907.02 Testimony by experts

(1) 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 the proponent demonstrates to the court that it is more likely than not that the testimony is based upon sufficient facts or data, the testimony is the product of reliable principles and methods, and the witness’s opinion reflects a reliable application of the principles and methods to the facts of the case. (2) Notwithstanding sub. (1), the testimony of an expert witness may not be admitted if the expert witness is entitled to receive any compensation contingent on the outcome of any claim or case with respect to which the testimony is being offered.

Wis. Stat. § 907.03 Bases of opinion testimony by experts

The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence in order for the opinion or inference to be admitted. Facts or data that are otherwise inadmissible may not be disclosed to the jury by the proponent of the opinion or inference unless the court determines that their probative value in assisting the jury to evaluate the expert’s opinion or inference substantially outweighs their prejudicial effect.

Wis. Stat. § 907.04 Opinion on ultimate issue

Testimony in the form of an opinion or inference otherwise admissible is not objectionable because it embraces an ultimate issue to be decided by the trier of fact.

Wis. Stat. § 907.05 Disclosure of facts or data underlying expert opinion

The expert may testify in terms of opinion or inference and give the reasons therefor without prior disclosure of the underlying facts or data, unless the judge requires otherwise. The expert may in any event be required to disclose the underlying facts or data on cross-examination.

Wis. Stat. § 907.06 Court appointed experts

(1) Appointment. The judge may on the judge’s own motion or on the motion of any party enter an order to show cause why expert witnesses should not be appointed, and may request the parties to submit nominations. The judge may appoint any expert witnesses agreed upon by the parties, and may appoint witnesses of the judge’s own selection. An expert witness shall not be appointed by the judge unless the expert witness consents to act. A witness so appointed shall be informed of the witness’s duties by the judge in writing, a copy of which shall be filed with the clerk, or at a conference in which the parties shall have opportunity to participate. A witness so appointed shall advise the parties of the witness’s findings, if any; the witness’s deposition may be taken by any party; and the witness may be called to testify by the judge or any party. The witness shall be subject to cross-examination by each party, including a party calling the expert witness as a witness. (2) Compensation. Expert witnesses so appointed are entitled to reasonable compensation in whatever sum the judge may allow. The compensation thus fixed is payable from funds which may be provided by law in criminal cases and cases involving just compensation under ch. 32. In civil cases the compensation shall be paid by the parties in such proportion and at such time as the judge directs, and thereafter charged in like manner as other costs but without the limitation upon expert witness fees prescribed by s. 814.04 (2). (3) Disclosure of appointment. In the exercise of discretion, the judge may authorize disclosure to the jury of the fact that the court appointed the expert witness. (4) Parties’ experts of own selection. Nothing in this rule limits the parties in calling expert witnesses of their own selection. (5) Appointment in criminal cases. This section shall not apply to the appointment of experts as provided by s. 971.16.

Wis. Stat. § 907.07 Reading of report by expert

An expert witness may at the trial read in evidence any report which the witness made or joined in making except matter therein which would not be admissible if offered as oral testimony by the witness. Before its use, a copy of the report shall be provided to the opponent.
Source: WI Statutes (Official Site)URL: https://docs.legis.wisconsin.gov/statutes/statutes/907Fetched: 10/7/2026