Expert witnesses may base opinions on facts or data that the expert “has been made aware of or personally observed.” Fed.R.Evid. 703. If the facts and data relied upon are the sort that experts in that field would reasonably rely on, then those facts “need not be admissible for the opinion to be admitted." Id. Accordingly, experts may base their opinions on otherwise-inadmissible information, such as hearsay, so long as the information is the sort reasonably relied upon in the experts' field.
The purpose of this rule is largely practical: experts generally base their opinions on information which, to be admissible in court, would entail “the expenditure of substantial time in producing and examining various authenticating witnesses.” Fed.R.Evid. 703 advisory committee's note. Because experts may use their past experience and professional judgment to make critical decisions on the basis of such information outside of court, Rule 703 was intended “to bring the judicial practice into line with the practice of the experts themselves when not in court." Id. Courts nevertheless must serve a gate-keeping function with respect to Rule 703 opinions to ensure “the expert isn't being used as a vehicle for circumventing the rules of evidence.” In re: James Wilson Assocs., 965 F.2d 160, 173 (7th Cir. 1992). Rule 703 “was not intended to abolish the hearsay rule and to allow a witness, under the guise of giving expert testimony, to in effect become the mouthpiece of the witnesses on whose statements or opinions the expert purports to base his opinion.” Loeffel Steel Prods., Inc. v. Delta Brands, Inc., 387 F.Supp.2d 794, 808 (N.D. Ill. 2005). The rule “was never intended to allow oblique evasions of the hearsay rule." Id.
Factory Mutual Insurance Co. v. Alon USA LP, 2013 WL 257134 (5th Cir. 2013)