The most significant theoretical problem with retrograde extrapolation is the use by the state's expert of Widmark's "r" and "beta" factors. These assumed values used in computing estimations of an individual's drinking pattern are impressive to a jury but very susceptible to cross-examination. The following materials, presented at the National College for DUI Defense's annual Summer Session at Harvard Law School, represents an excellent explanation of the problem — and the solution. They are reprinted with the kind permission of Michael Snure, of Kirkconnell, Lindsey, Snure & Henson in Winter Park, Florida.
Retrograde extrapolation is often attempted by prosecution experts. Simply stated, an attempt to retrograde extrapolate a person's breath value is an attempt to determine or estimate what the person's breath test value would have been at an earlier time (presumably the time of the driving) based on a subsequent test. Because a person is rarely tested within a few minutes of driving, the subsequent breath test only gives a value for what the person's breath test value was at the time they were tested. Retrograde extrapolation is a flawed method employed in an attempt to estimate what the person's breath test value would have been at the time of driving.
Many experts now agree that retrograde extrapolation is so imprecise or can be stated only with respect to such a broad range that it is virtually impossible or useless in the forensic setting. However, certain facts must be known before even a bastardized attempt to extrapolate can be made. The minimum factors which must be known are:
Assuming that those variables are known an expert will then have to resort to the use of two very generalized assumptions which can be attacked.
The entire basis for retrograde extrapolation began following the research of' Professor Widmark in the 1930s. The mechanisms for the absorption and elimination of alcohol from the body are fairly well understood. For a period of time after drinking, an individual's blood alcohol level may continue to rise, remain constant, or fall depending on whether the rate of absorption of the alcohol is greater than, equal to, or less than the elimination. This single factor seriously impairs the efforts to estimate from a single breath test measurement the breath test value at some prior time, or the amount of alcohol which must have been consumed to give a particular breath value.
Professor Widmark's research was originally published in German. Very few prosecution experts have ever read the entire text and are probably unfamiliar with the research. It should be remembered that Professor Widmark studied blood alcohol content as it changed with time after drinking. His subjects were required to consume a known amount of alcohol on an empty stomach after which blood samples were taken for analysis every fifteen minutes for three hours. The results were analyzed and plots were made of the blood alcohol content over the course of time. This was the genesis of the commonly referred to "blood alcohol curve."
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