California Civil Jury Instructions (CACI)

220. Experts—Questions Containing Assumed Facts

The law allows expert witnesses to be asked questions that are based on assumed facts. These are sometimes called “hypothetical questions.”

In determining the weight to give to the expert’s opinion that is based on the assumed facts, you should consider whether the assumed facts are true.

New September 2003

Directions for Use

Juries may be instructed that they should weigh an expert witness’s response to a hypothetical question based on their assessment of the accuracy of the assumed facts in the hypothetical question. (Treadwell v. Nickel (1924) 194 Cal. 243, 263–264 [228 P. 25].)

For an instruction on expert witnesses generally, see CACI No. 219, Expert Witness Testimony. For an instruction on conflicting expert testimony, see CACI No. 221, Conflicting Expert Testimony.

Sources and Authority

  • The value of an expert’s opinion depends on the truth of the facts assumed. (Richard v. Scott (1978) 79 Cal.App.3d 57, 63 [144 Cal.Rptr. 672].)
  • “Generally, an expert may render opinion testimony on the basis of facts given ‘in a hypothetical question that asks the expert to assume their truth.’ ” (People v. Vang (2011) 52 Cal.4th 1038, 1045 [132 Cal.Rptr.3d 373, 262 P.3d 581].)
  • Hypothetical questions must be based on facts that are supported by the evidence: “It was decided early in this state that a hypothetical question to an expert must be based upon facts shown by the evidence and that the appellate court will place great reliance in the trial court’s exercise of its discretion in passing upon a sufficiency of the facts as narrated.” (Hyatt v. Sierra Boat Co. (1978) 79 Cal.App.3d 325, 339 [145 Cal.Rptr. 47].)
  • “A hypothetical question need not encompass all of the evidence. ‘It is true that “it is not necessary that the question include a statement of all the evidence in the case. The statement may assume facts within the limits of the evidence, not unfairly assembled, upon which the opinion of the expert is required, and considerable latitude must be allowed in the choice of facts as to the basis upon which to frame a hypothetical question.” On the other hand, the expert’s opinion may not be based “on assumptions of fact without evidentiary support [citation], or on speculative or conjectural factors . . . .” ’ ” (People v. Vang, supra, 52 Cal.4th at p. 1046, internal citation omitted.)
  • Hypothetical questions should not omit essential material facts. (Coe v. State Farm Mutual Automobile Insurance Co. (1977) 66 Cal.App.3d 981, 995 [136 Cal.Rptr. 331].)

  • The jury should not be instructed that they are entitled to reject the entirety of an expert’s opinion if a hypothetical assumption has not been proven. Rather, the jury should be instructed “to determine the effect of that failure of proof on the value and weight of the expert opinion based on that assumption.” (Lysick v. Walcom (1968) 258 Cal.App.2d 136, 156 [65 Cal.Rptr. 406].)
  • “The jury still plays a critical role in two respects. First, it must decide whether to credit the expert’s opinion at all. Second, it must determine whether the facts stated in the hypothetical questions are the actual facts, and the significance of any difference between the actual facts and the facts stated in the questions.” (People v. Vang, supra, 52 Cal.4th at pp. 1049–1050.)

Secondary Sources

3 Witkin, California Evidence (4th ed. 2000) Presentation at Trial, §§ 194–201

Jefferson, California Evidence Benchbook (3d ed. 1997) § 29.43, pp. 609–610

3A California Trial Guide, Unit 60, Opinion Testimony, §§ 60.05, 60.50–60.51 (Matthew Bender)

California Products Liability Actions, Ch. 4, The Role of the Expert, § 4.03 (Matthew Bender)

48 California Forms of Pleading and Practice, Ch. 551, Trial, § 551.70 (Matthew Bender)