Federal Rules of Evidence 703
In Wilson v. Clark, 84 Ill. 2d 186, 417 N.E.2d 1322, 49 Ill. Dec. 308 (1981), our supreme court adopted the standard for the use of expert opinion testimony at trial contained in Federal Rules of Evidence 703 and 705. Under Rule 703, an expert may base his or her opinion upon facts that are not in evidence if those facts are of a type reasonably relied upon by experts in the particular field. Wilson, 84 Ill. 2d at 193, 417 N.E.2d at 1326.
The advent of Rule 703 did not, however, create an exception to the hearsay rule as the underlying facts or data relied upon by the expert are not admitted for their truth. City of Chicago v. Anthony, 136 Ill. 2d 169, 185, 554 N.E.2d 1381, 1388, 144 Ill. Dec. 93 (1990).
Rather, they are admitted "for the limited purpose of explaining the basis for the expert witness' opinion." People v. Anderson, 113 Ill. 2d 1, 12, 495 N.E.2d 485, 489, 99 Ill. Dec. 104 (1986).
In Wilson v. Clark, 84 Ill. 2d 186, 417 N.E.2d 1322, 49 Ill. Dec. 308 (1981), our supreme court adopted the then-existing Federal Rule of Evidence 703, holding that an expert may offer an opinion based upon facts not in evidence if those facts are "of a type reasonably relied upon by experts in the particular field." People v. Raney, 324 Ill. App. 3d 703, 706, 756 N.E.2d 338, 258 Ill. Dec. 356 (2001), citing Wilson, 84 Ill. 2d at 193. Where expert testimony is based upon an electronic or mechanical device, however, the expert must provide some foundational proof that the device is functioning properly at the time it was used. Raney, 324 Ill. App. 3d at 706 (proper foundation was lacking where expert failed to provide any testimony that the machine used was calibrated and working properly or how she knew the results were accurate).
Raney recognized, however, that it may not be feasible to require an expert to personally test the instrument relied on for making relevant determinations. Raney, 324 Ill. App. 3d at 710.
The hearsay rule traditionally prohibits the admission of out-of-court statements where offered as an assertion to show the truth of the matter asserted therein, thus resting for its value upon the credibility of the out-of-court asserter. People v. Carpenter, 28 Ill. 2d 116, 121, 190 N.E.2d 738, 741 (1963).
See also C. McCormick, Handbook of Evidence 225 (1954).
The rule is not implicated, however, where statements otherwise inadmissible as hearsay are used by experts for the purpose of explaining the bases for their opinions. People v. Jones, 374 Ill. App. 3d 566, 579-80, 871 N.E.2d 823, 834-35, 313 Ill. Dec. 96 (2007).