Legal Memorandum: Admissibility of expert testimony

Issue: Under the Illinois Rules of Evidence, when is a witness qualified to testify as an expert?

Illinois Rule 702 covers situations where a witness may testify as an expert.  The Rule provides that

[i]f scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise. Where an expert witness testifies to an opinion based on a new or novel scientific methodology or principle, the proponent of the opinion has the burden of showing the methodology or scientific principle on which the opinion is based is sufficiently established to have gained general acceptance in the particular field in which it belongs.

Ill. R. Evid. 702 (2015).

In State v. Prather, the trial court erred in barring the evidence on the basis of the Frye standard:

¶ 17 In this case, the first step of the Frye standard is not met, because the technology that the State sought permission for B.R. to testify about-a home pregnancy test-is neither new nor novel. The second step, the general-acceptance test, is applied only when the proposed evidence is "based on a new or novel scientific methodology or principle." Ill. R. Evid. 702 (eff. Jan. 1, 2011). Here, the basic principle involved, that human chorionic gonadotropin (hCG) in the urine is an early sign of pregnancy, has been known since the 1920s; many of the methodologies that allowed easy detection of […]

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