Expert Witnesses

Federal Rules of Evidence
http://www.law.cornell.edu/rules/fre/index.html#article_vii

Rule 701. Opinion Testimony by Lay Witnesses

If the witness is not testifying as an expert, the witness testimony in the form of opinions or inferences is limited to those opinions or inferences which are (a) rationally based on the perception of the witness, and (b) helpful to a clear understanding of the witness' testimony or the determination of a fact in issue, and (c) not based on scientific, technical, or other specialized knowledge within the scope of Rule 702 .

Rule 702. Testimony by Experts

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.

Rule 703. 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 shall 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 substantially outweighs their prejudicial effect.

Rule 704. Opinion on Ultimate Issue

(a) Except as provided in subdivision (b), 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.

(b) No expert witness testifying with respect to the mental state or condition of a defendant in a criminal case may state an opinion or inference as to whether the defendant did or did not have the mental state or condition constituting an element of the crime charged or of a defense thereto. Such ultimate issues are matters for the trier of fact alone.

Rule 705. Disclosure of Facts or Data Underlying Expert Opinion

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

California Evidence Code 

http://www.leginfo.ca.gov/cgi-bin/calawquery?codesection=evid&codebody=&hits=20 

720. Expert Witness 

(a) A person is qualified to testify as an expert if he has special knowledge, skill, experience, training, or education sufficient to qualify him as an expert on the subject to which his testimony relates.  Against the objection of a party, such special knowledge, skill, experience, training, or education must be shown before the witness may testify as an expert.

(b) A witness' special knowledge, skill, experience, training, or education may be shown by any otherwise admissible evidence, including his own testimony.

721. Expert Witness Cross-Examination

(a) Subject to subdivision (b), a witness testifying as an expert may be cross-examined to the same extent as any other witness and, in addition, may be fully cross-examined as to (1) his or her qualifications, (2) the subject to which his or her expert testimony relates, and (3) the matter upon which his or her opinion is based and the reasons for his or her opinion.

(b) If a witness testifying as an expert testifies in the form of an opinion, he or she may not be cross-examined in regard to the content or tenor of any scientific, technical, or professional text, treatise, journal, or similar publication unless any of the following occurs:

   (1) The witness referred to, considered, or relied upon such publication in arriving at or forming his or her opinion.

   (2) The publication has been admitted in evidence.

   (3) The publication has been established as a reliable authority or admission of the witness or by other expert testimony or by judicial notice.

   If admitted, relevant portions of the publication may be read into evidence but may not be received as exhibits.

722.  Court Appointment; Compensation to Expert

(a) The fact of the appointment of an expert witness by the court may be revealed to the trier of fact.

(b) The compensation and expenses paid or to be paid to an expert witness by the party calling him is a proper subject of inquiry by any adverse party as relevant to the credibility of the witness and the weight of his testimony.

723.     Limiting Expert Witnesses

The court may, at any time before or during the trial of an action, limit the number of expert witnesses to be called by any party.

801.  Expert Witness Testimony

If a witness is testifying as an expert, his testimony in the form of an opinion is limited to such an opinion as is:

        (a) Related to a subject that is sufficiently beyond common experience that the opinion of an expert would assist the trier of fact; and

         (b) Based on matter (including his special knowledge, skill, experience, training, and education) perceived by or personally known to the witness or made known to him at or before the hearing, whether or not admissible, that is of a type that reasonably may be relied upon by an expert in forming an opinion upon the subject to which his testimony relates, unless an expert is precluded by law from using such matter as a basis for his opinion.

802.  Expert Witness May Testify about Basis for Testimony

A witness testifying in the form of an opinion may state on direct examination the reasons for his opinion and the matter (including, in the case of an expert, his special knowledge, skill, experience, training, and education) upon which it is based, unless he is precluded by law from using such reasons or matter as a basis for his opinion.  The court in its discretion may require that a witness before testifying in the form of an opinion be first examined concerning the matter upon which his opinion is based.

803.     Court May Exclude Testimony

The court may, and upon objection shall, exclude testimony in the form of an opinion that is based in whole or in significant part on matter that is not a proper basis for such an opinion.  In such

case, the witness may, if there remains a proper basis for his opinion, then state his opinion after excluding from consideration the matter determined to be improper.

804.  Testimony Based on Another’s Opinion

(a) If a witness testifying as an expert testifies that his opinion is based in whole or in part upon the opinion or statement of another person, such other person may be called and examined by any adverse party as if under cross-examination concerning the opinion or statement.

(b) This section is not applicable if the person upon whose opinion or statement the expert witness has relied is (1) a party, (2) a person identified with a party within the meaning of subdivision (d) of Section 776, or (3) a witness who has testified in the action concerning the subject matter of the opinion or statement upon which the expert witness has relied.

(c) Nothing in this section makes admissible an expert opinion that is inadmissible because it is based in whole or in part on the opinion or statement of another person.

(d) An expert opinion otherwise admissible is not made inadmissible by this section because it is based on the opinion or statement of a person who is unavailable for examination pursuant to this section.

805. Opinion on Ultimate Issue

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