Convicting the Average DUI Suspect
Information courtesy of Lawrence Taylor - DUIblog
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. Put another way, obtaining an accurate blood-alcohol reading in a DUI case is completely dependent on the validity of a number of assumptions. Unfortunately for the person being tested, these assumptions are usually incorrect: The person tested is rarely "average" in even one of these critical characteristics, let alone in all of them.
For example, all breath testing devices depend on the assumption that the ratio between alcohol in the exhaled breath and alcohol in the blood is 1 to 2100. In fact, the machine is designed to produce a reading based on that assumption; the accuracy of the reading is directly tied to the accuracy of the presumption. Yet, the actual ratio in any given individual can vary from less than 1:1300 to more than 1:3000. So a DUI suspect with a true blood-alcohol level of .08 but a breath-to-blood ratio of, say, 1:1700 would have a .10 reading on an "accurate" breath testing instrument.
Put simply, these machines do not test individuals. Rather, they test the same "average suspect" over and over again, but using the individual subject's breath.
Another example of the assumption of "averageness" can be found in urinalysis. When a DUI suspect's urine is analyzed for blood-alcohol, a presumption exists that there are 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 ratiothat is, it is based entirely on the ratio found in the average person. In fact, however, the actual ratio found in any given individual can vary greatly. And as the ratio is in error, so will be the final blood-alcohol reading.
Another example of this constant reliance on averages shows itself when the prosecutor in a DUI trial offers evidence of so-called "retrograde extrapolation", or guessing backwards. The blood-alcohol level at the time of testing is not relevant to the charge, of course, and