Testimony by one acknowledged to have special training and knowledge in a particular subject. Only testimony on the subject in which the witness is expert is considered expert testimony. An expert can testify to an opinion or inference if the following five conditions are met: (1) Specialized knowledge helpful to the fact finder is available; (2) Witness specially qualified; (3) Proper basis for the opinion; (4) Underlying data revealed or available; and (5) Reasonable degree of certainty. The opinion of an expert can be based on: (1) Facts personally observed; (2) Evidence adduced during trial; (3) A hypothetical question; and (4) Data conveyed by counsel or others. An expert needs to have a reasonable degree of certainty about his expert opinion in most jurisdictions. A large number require merely a high probability.
An expert must base her opinion either on facts personally observed or on hypotheses that find support in the evidence.' (George v. Bekins Van & Storage Co. (1949) 33 Cal.2d 834, 844 [205 P.2d 1037]. See also Kastner v. Los Angeles Metropolitan Transit Authority (1965) 63 Cal.2d 52, 58 [45 Cal.Rptr. 129, 403 P.2d 385]; Commercial Union Assur. Co. v. Pacific Gas & Electric Co. ...