The state of Illinois follows the Frye test for the admissibility of expert witness testimony. Donnellan v. First Student, Inc., 891 N.E.2d 463 (Ill. App. Ct. 2008). Under the Frye test, expert witness testimony is admissible if “the methodology underlying the opinion is ‘sufficiently established to have gained general acceptance in the particular field in which it belongs.’” Id. This test requires the court to analyze the method underlying the expert’s testimony and not on the expert’s conclusions from the testing and methodology. Id. Furthermore, general acceptance does not imply that the “methodology be accepted by unanimity, consensus, or even a majority of experts.” People v. Swart, 860 N.E.2d 1142 (Ill. App. Ct. 2006). The purpose of subjecting expert witness testimony to the Frye test is to ensure that invalid evidence is not permitted and relied on in cases. Id.

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