California Civil Jury Instructions (CACI) (2017)

220. Experts—Questions Containing Assumed Facts

Download PDF
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
67
0033
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 (5th ed. 2012) Presentation at Trial, §§ 208–215
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)
California Judges Benchbook: Civil Proceedings—Trial (2d ed.) § 8.92 (Cal CJER
2010)
CACI No. 220 EVIDENCE
68
0034