If a witness is not testifying as an expert, testimony in the form of an opinion is limited to one that is:
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
- the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
- the testimony is based on sufficient facts or data;
- the testimony is the product of reliable principles and methods; and
- the expert has reliably applied the principles and methods to the facts of the case.
- Specialized knowledge is knowledge that is not particularized. The knowledge applies to things that one does not have experience with.
- The expert's testimony must be helpful to the jury. It cannot confuse or invade the providence of the jury. (Such as addressing witness credibility or saying that something was negligent or another legal term)
An expert may base an opinion on facts or data in the case that the expert has been made aware of or personally observed. If experts in the particular field would reasonably rely on those kinds of facts or data in forming an opinion on the subject, they need not be admissible for the opinion to be admitted. But if the facts or data would otherwise be inadmissible, the proponent of the opinion may disclose them to the jury only if their probative value in helping the jury evaluate the opinion substantially outweighs their prejudicial effect.
- An expert can provide his opinion on things he did not perceive, even things provided at trial. (Like hypotheticals)
- In General — Not Automatically Objectionable. An opinion is not objectionable just because it embraces an ultimate issue.
- Exception. In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone.
Unless the court orders otherwise, an expert may state an opinion — and give the reasons for it — without first testifying to the underlying facts or data. But the expert may be required to disclose those facts or data on cross-examination.