The Lawyer's Guide to DUI Defense - The Widmark Factor
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The other insidious assumption in the retrograde extrapolation calculation is the beta factor which is commonly referred to as a rate of elimination. Widmark's research indicated that the arithmetic average for rate of elimination was 0.015% per hour. There is no basis on which to assume that everyone has essentially the same beta factor or elimination rate. Elimination rates have been studied and found to vary from values of .006% per hour to as high as .04% per hour. Moreover, the research does not find a tight grouping or bunching of elimination rates around any one average value.
An example of the amount of error which may be introduced to an extrapolation equation can be found by assigning an elimination rate of .015% for someone whose actual rate is .006%. This will result in a 150% overestimation of the person's blood alcohol level at an earlier time. Furthermore, any such calculation always involves the assumption that the person's blood alcohol level was declining throughout the entire period in question. This is often never known. The expert usually will perform retrograde extrapolation without any demonstrative knowledge of whether the time of the incident blood alcohol content was before or after the person's peak. It is almost universally assumed that the time in question occurred at or after a person reached the peak alcohol absorption. This assumption is baseless.
The outstanding consideration which must be established in cross-examining any expert who attempts to retrograde extrapolate is that the averaged values for r and beta are merely arithmetic averages. They are the result of adding values and dividing by the number of subjects. The range of values is wide and not tightly grouped or packed around any one value. Therefore while the results can be averaged for groups, no value can be predicted for any individual within that group.
Finally, the acknowledged authoritative prosecution expert, Kurt Dubowski, has essentially denounced the use of retrograde extrapolation because of the hazards inherent in the assumptions which must be made in the calculations.
Mr. Snure then offers the following checklist to be used in assessing prosecutorial evidence of retrograde extrapolation:
- Does the expert know enough about the drinking pattern and the individual?
- Does the expert recognize that the r factor is not a fixed number but varies across a broad range and is not applicable to any "average man"?
- There is a wide range of r values with considerable distribution among the range even in small population samples and the values of r for men are higher than for women.
- An r value cannot be arbitrarily assigned to someone, male or female, for purposes of making a reliable calculation.
- The r values as originally obtained by Widmark were not obtained in any social drinking situation but were rather obtained in a clinical setting involving consumption of alcohol in one large dose on an empty stomach.
- The beta factor is not a fixed number that could be applied to any individual person or average man.
- The beta factors cover a wide range with substantial distribution across the range in both large and small populations.
- The beta value cannot be assigned to any particular individual for the purpose of making any reliable calculations of' the amount of alcohol consumed over time nor for the purpose of' reliably estimating someone's blood alcohol content at a prior time.
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