One of the greatest sources of error in blood-alcohol testing is the false assumption that the individual tested is perfectly average in certain critical physiological traits. Put another way, obtaining an accurate blood-alcohol reading is completely dependent on the validity of a number of scientific 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 BAC 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. The actual ratio in any given individual can vary from 1:1300 to 1:3000, or even more widely. Thus a person with a true blood-alcohol level of .08 but a breath-to-blood ratio of 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 average person over and over again, but using the subject's breath.
Yet another example of the assumption of "averageness" can be found in urinalysis. When a subject'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. 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 offers evidence of retrograde extrapolation (assumption that blood-alcohol levels drop after you stop drinking). The blood-alcohol level at the time of testing is not relevant to the charge, of course and so the state will offer evidence to show what the level was when the defendant was driving. This is commonly done by computing the earlier blood-alcohol level by estimating how much alcohol had been eliminated or "burned off" in the time between driving and testing. But this requires two assumptions:
The blood-alcohol level was declining and the rate of elimination is known.
That the "burn-off" rate was .015 percent per hour.
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