Retrograde Extrapolation Alive and Well

I wrote here about the court of appeals’ decision in State v. Davis, __ N.C. App. __, 702 S.E.2d 507 (2010), granting the defendant a new trial on second degree murder, impaired driving and other charges arising from a fatal hit-and-run committed by the defendant after she had been drinking. Davis determined that expert testimony as to the defendant’s blood alcohol concentration at the time of the crash was improper and prejudicial where that testimony was founded solely on the odor of alcohol on defendant’s breath more than ten hours after the accident.

Some wondered whether Davis portended the court of appeals’ inclination to reconsider the propriety of expert testimony based upon retrograde extrapolation, a methodology used to estimate a person’s alcohol concentration at some earlier point in time based upon a later reported alcohol concentration.  After all, courts in other states have viewed such testimony with skepticism. The Texas Court of Criminal Appeals in Mata v. Texas, 46 S.W.3d 902 (Tex. Crim. App. 2001) (en banc), summarized its view of the limitations of retrograde extrapolation as follows:

Initially, we recognize that even those who believe retrograde extrapolation is a reliable technique have utilized it only if certain factors are known, such as the length of the drinking spree, the time of the last drink, and the person’s weight. . . . In addition, there appears to be general disagreement on some of the fundamental aspects of the theory, such as the accuracy of Widmark’s formulas . . . whether a standard elimination rate can be reliably applied to a given subject . . . and the effect that food in the stomach has on alcohol absorption. . . . Nevertheless, given the studies, other concepts seem indisputable, including that multiple tests will increase the ability to plot a subject’s BAC curve, a test nearer in time to the time of the alleged offense increases the ability to determine the subject’s offense-time BAC, and the more personal information known about the subject increases the reliability of an extrapolation. . . . . We believe that the science of retrograde extrapolation can be reliable in a given case. The expert’s ability to apply the science and explain it with clarity to the court is a paramount consideration. In addition, the expert must demonstrate some understanding of the difficulties associated with a retrograde extrapolation. He must demonstrate an awareness of the subtleties of the science and the risks inherent in any extrapolation. Finally, he must be able to clearly and consistently apply the science.

Id. at 916-917. See also United States v. Dubois, 645 F.2d 642 (8th Cir. 1981) (evaluating sufficiency of the evidence, “emphasiz[ing] that this was a criminal trial,” and finding expert’s extrapolation based on a test taken two and one-half hours after the accident and after the undisputed consumption of an unknown amount of beer insufficient to establish intoxication at time of accident); A.W. Jones, Disposition and Fate of Ethanol in the Body, in Medical-Legal Aspects of Alcohol 95 (James C. Garriott ed., 4th ed. 2003) (“Requests to back extrapolate BAC from time of sampling to time of driving are common in DUI litigation although this is a dubious practice with many variables to consider.”); Cf. Weinstein v. Siemens, 2010 WL 4825205 (E.D. Mich. Nov. 22, 2010) (finding retrograde extrapolation testimony from expert with Ph.D. in toxicology admissible to prove driver’s alcohol concentration at the time of accident where expert relied on three consecutive blood draws to determine the driver’s rate of elimination and applied that rate in its extrapolation analysis to conclude that driver’s alcohol concentration was in the range of .36 to .39, depending upon whether his alcohol concentration was increasing or decreasing).

State v. Green, decided by the court of appeals March 1, 2011, indicates that any such reading of the tea leaves was inaccurate. The defendant in Green was charged with impaired driving based upon an accident that occurred in Raleigh around 8 p.m. on December 14, 2006. A witness who spoke to defendant at the scene through the passenger side window of her car reported that the defendant’s “‘eyes were kind of half shut and glazed looking, and he just said:  Huuuuh?’” (Slip op. at 2.) The accident was reported via a 911 call at 8:06 p.m., but the defendant drove away before police arrived. A police officer went to defendant’s home at 9:38 p.m. The officer described defendant as “‘sluggish’” and “‘slow’” and said he could “‘smell the odor of mouthwash with a moderate to strong odor of alcohol coming through that.’” Id. The defendant first denied having had anything to drink, then said he had one glass of wine. Later the defendant said he had consumed five glasses of wine after arriving home at 7:15 p.m.

Defendant was arrested for driving while impaired. He submitted to a breath test at 11:28 p.m., which revealed an alcohol concentration of 0.19.

Paul Glover, head of the Forensic Tests for Alcohol branch of the State Department of Health and Human Services, testified at trial as an expert witness in physiology and pharmacology. Glover testified as to his calculation of defendant’s blood alcohol concentration (“BAC”) at 8:06 p.m.—the time of 911 call—based on the following factors: the breath test results, the time that had elapsed since the time of the call, “the rate at which a human body eliminates alcohol,” defendant’s size and gender, “and defendant’s assertions to Officer Larsen that he consumed as little as no alcohol to as much as five glasses of wine.”

Glover testified that that if defendant consumed no more alcohol after the 911 call, his alcohol concentration at 8:06 p.m. would have been 0.24. This sort of retrograde extrapolation testimony is akin to that approved in earlier North Carolina cases. See, e.g., State v. Taylor, 165 N.C. App. 750 (2004); State v. Catoe, 78 N.C. App. 167 (1985). That is, it is calculated from a single reading taken some hours after the incident, assuming an average elimination rate and that the defendant was in the elimination versus the absorption phase. The reliability of such a calculation may be questioned for the reasons noted in Mata, but its admission is consistent with state precedent.

Glover further testified that, presuming defendant had one glass of wine after 8:06 p.m., his BAC at the time of the accident was 0.23. And, if the defendant drank, as he claimed, five glasses of wine (assuming 5-ounce glasses and 12 percent alcohol) after the accident, his BAC at 8:06 p.m. was 0.19. Finally, Glover testified that assuming defendant drank no alcohol before 8:06 p.m., he would have to have consumed 88 ounces of wine (again assuming 5 ounce glasses and 12 percent alcohol) by 9:38 p.m. to have reached a BAC of 0.19. This calculation requires estimation of the rate at which the defendant’s body absorbed alcohol, a rate that depends upon many factors, including the speed with which alcohol is consumed and whether the defendant had an empty stomach. See this post and Jones, supra, at 51 (noting that the “variable factors involved in the disposition of ethanol in the body . . . make it easy to appreciate the reason for large inter- and intra-individual variations in the BAC profiles observed in experimental studies”).

Given our appellate courts’ continued endorsement of expert testimony based on retrograde extrapolation and related theories, I wonder what the best defense strategies are for pointing out the limitations of the methodology. Is cross-examination alone adequate or does this call for a battle of the experts?

2 thoughts on “Retrograde Extrapolation Alive and Well”

  1. It is likely that the best defense – at present – is “a battle of the experts.” The inherent scientific weakness of retrograde extrapolation, as inflicted in North Carolina, appears to be the complete absence of measures of quality when the result is pronounced. Such measures of quality include accuracy and precision. Regrettably as well as understandably, these concepts do not play well to fact finders. Based upon my experience, it appears unlikely that cross examination of the advocate of retrograde extrapolation about such quality measures would be fruitful. Ignorance can be bliss.

    Howerton does provide some guidance to the practitioner who might wish to challenge the reliability of retrograde extrapolation. At its most fundamental level, chemical analysis relies upon comparisons. Howerton appears to identify as a reliability factor the use of comparisons. For the chemical analyst, a big question attending any measurement is a comparison, specifically, how well does the measurement result compare to a known standard. Our retrograde extrapolation avoids this fundamental comparison. Indeed, it avoids any comparison. (The number I calculate by extrapolation is what it is, because I say it is; don’t ask me to say how good it is. See also Proverbs 11:1 (providing early commentary on the importance of calibration standards in the face of a simple analytical machine that, in the wrong hands, could prove an abomination, even back then).)

  2. Has anyone used Dr. Andrew Mason as an expert wintess in retrograde extrapolation cases? If so, comments and any contact information would be grateful.


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