Case Summary: Foundational Requirements for Expert Opinion Testimony


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Source: Pamela A.W. King, JD, Jurisprudence Section Secretary; Article by Donald J. Ramsell, JD

On May 1 in honor of National Law Day, President Betty Layne DesPortes announced a newsfeed project soliciting articles discussing significant or interesting cases concerning the use of science in the justice system. All sections were encouraged to submit cases that highlight the use of proper scientific principles or the application of legal standards for the admission of scientific evidence. The following case summary discusses the need to establish an adequate foundation for the admission of expert opinion testimony in the context of blood alcohol concentration (BAC) retrograde extrapolation calculations.

In the Illinois case of People v. Floyd, the defendant was charged with Aggravated DUI.1 During the trial, the State introduced expert witness testimony on a “retrograde extrapolation” calculation in an attempt to demonstrate that the defendant’s Blood Alcohol Concentration (BAC) was at or above 0.08 at the time of her arrest. Retrograde extrapolation is premised on the theory that a person’s BAC, derived from a breath or blood test at a particular time, can be extrapolated back to an allegedly higher BAC that existed at the time of a prior incident. Law enforcement officers administered a breath test to the defendant at 10:30 p.m., which registered her BAC at 0.069. At trial, the State produced a forensic toxicologist as an expert witness. The expert witness testified that, after conducting a retrograde extrapolation calculation, he determined the defendant’s BAC at 9:10 p.m. (the time of driving) was between 0.082 and 0.095. The defendant was convicted.

On appeal, the defendant contended that the trial court erred by allowing the expert witness’ testimony on retrograde extrapolation when the expert did not have information necessary to conduct a reliable calculation. The expert witness had explained that conducting a retrograde extrapolation calculation is possible because a person eliminates alcohol at a fixed rate of between 0.01 grams and 0.02 grams per deciliter of blood per hour. He explained that two conditions must be met for the calculation to be valid. First, the person metabolizes alcohol at the normal rate. Second, the person is in the postabsorption phase—that is, the person is no longer absorbing alcohol and is in the elimination phase—when the breath test is administered. The expert witness explained that, because the body absorbs alcohol primarily through the small intestine, a person’s absorption rate will vary depending on many factors. These factors include the type of food the person has eaten, the type of alcohol consumed, and the length of time during which the drinking occurred. He testified that after a person’s last drink, a person can take from 15 minutes to 90 minutes or more to enter the postabsorption phase.

On cross-examination, the expert witness acknowledged that he did not know what the defendant had eaten that night, how long she had been drinking, or what type of alcohol she consumed. He admitted that he did not attempt to determine when the defendant had entered the elimination phase, but rather assumed that she was in the elimination phase at 9:10 p.m. He also explained that if a person had not consumed any alcohol since 7:30 p.m., he would be “quite confident” that the person was in the elimination phase by 9:10 p.m.

The appellate court reversed, finding that the expert’s opinion was improperly admitted. “A retrograde extrapolation calculation based on a single breath test, and when many of the factors necessary to determine whether the defendant was in the elimination phase are unknown, is insufficient to provide a reliable calculation and invites the jury to determine guilt on an improper basis. Based on the specific circumstances presented in this case, we believe that the prejudicial effect of the retrograde extrapolation calculation substantially outweighed its probative value and that the trial court abused its discretion in admitting it.”

  1. People v. Floyd, 2014 IL App (2d) 120507, 1-2, 11 N.E.3d 335, 336