Taimoor Rana is seasoned criminal defense attorney specializing in DUI related cases. He shares his expert legal opinion here « Why every DUI suspect is considered as “Average”?
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- One of the greatest sources of error in breath-alcohol testing is the consistently recurring fallacy that the individual tested is perfectly average in certain critical physiological traits.
Explaining this in other way, there are a number of assumptions on the validity of which an accurate blood-alcohol reading is obtained and its accuracy is completely dependent on the validity of those assumptions.
Unluckily, these assumptions are usually incorrect for the person being tested: It rarely happens that the person tested is “average” in even one of these critical characteristics.
‘Average’ Factor in Breath Testing Machine
For instance, it is assumed that the ratio between alcohol in the exhaled breath and alcohol in the blood is 1 to 2100. All breath testing devices depend on this assumption. The fact is that, the machine is designed such that it produce a reading based on that assumption; the accuracy of the reading is directly connected to the accuracy of the presumption.
Variation in Actual Ratio from person to person
Yet, there is a variation in the actual ratio in any given individual.
It varies from less than 1:1300 to more than 1:3000. So a DUI suspect having a true blood-alcohol level of .08 but a breath-to-blood ratio of, for instance, 1:1700 would have a .10 reading on an “accurate” breath testing instrument.
Breath Machines test the same “average suspect” every time
Simply explaining, these machines do not test individuals. Rather than doing that, they test the same “average suspect” every time, but using the individual subject’s breath.
‘Average’ Factor in Urinalysis
The assumption of “averageness” can also be explained by the example regarding urinalysis. When a DUI suspect’s urine is analyzed for blood-alcohol, there is a presumption that exists there and that is, 1.3 parts of alcohol in the bladder’s urine for every 1 part of alcohol in the blood.
This 1:1.3 ratio is as fallacious as the 1:2100 ratio. What it means is that, it is based entirely on the ratio that is found in the average person.
It is a fact that there is a huge variation in the actual ratio found in any given individual. As the ratio that is taken is inaccurate, so will be the final blood-alcohol reading.
‘Average’ Factor in “extrapolating backward”
There is another example of this constant reliance on averages. This is showed when evidence of so-called “retrograde extrapolation”, or guessing backwards is offered by the prosecutor in a DUI trial. Of course, the blood-alcohol level at the time of testing is not relevant to the charge, so the state will offer evidence to show the blood-alcohol level at the time when the defendant was driving.
This is commonly done by “extrapolating” backward, that is, the earlier blood-alcohol level is computed by making an estimate that how much alcohol had been eliminated or “burned off” during the period between driving and testing.
Two Assumptions are required for “extrapolating” backward
For extrapolating” backward two assumptions are required.First is that the blood-alcohol level was declining and the second one is that rate of elimination is known. The second assumption further involves the assumption that the “burn-off” rate was .015% per hour (sometimes the assumed rate is .02 percent).
How does the prosecution know with surety that the defendant was eliminating (assuming he was eliminating rather than absorbing) at 0.15% and not at .005% or .3% ?
The answer is quite simply, the prosecution does not know: They are only assuming that the defendant eliminates at the average rate. And, of course, if there is an error in such an assumption then the extrapolation will be wrong automatically.
So why is that the state presume facts that are clearly wrong?
Answer is Simple. It is convenient: it makes easy to prosecute and convict a person for DUI.
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