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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:
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