California Evidence Code
If a witness is not testifying as an expert, his testimony in the form of an opinion is limited to such an opinion as is permitted by law, including but not limited to an opinion that is:
(a) Rationally based on the perception of the witness; and
(b) Helpful to a clear understanding of his 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.
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.
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.
(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.
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.