“According to Evidence Code section 801, subdivision (b), expert testimony may be excluded if it is not “[b]ased on matter . . . 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 . . . .” (Ibid.) As our Supreme Court has explained, “under Evidence Code section 801, the trial court acts as a gatekeeper to exclude speculative or irrelevant expert opinion. . . . ‘[T]he expert’s opinion may not be based “on assumptions of fact without evidentiary support [citation], or on speculative or conjectural factors. . . . [¶] Exclusion of expert opinions that rest on guess, surmise or conjecture [citation] is an inherent corollary to the foundational predicate for admission of the expert testimony: will the testimony assist the trier of fact to evaluate the issues it must decide?” ‘ ” (Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747, 770 (Sargon).) “[A]n expert’s opinion that something could be true if certain assumed facts are true, without any foundation for concluding those assumed facts exist in the case before the jury, does not provide assistance to the jury . . . .” (Jennings v. Palomar Pomerado Health Systems, Inc. (2003) 114 Cal.App.4th 1108, 1117).”
The same holds for speculation and conjecture of mediators and evaluators.