One of the greatest sources of error in blood-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 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 partition ratio between alcohol in the exhaled breath and alcohol in the blood is 1 to 2100. In fact, the breath alcohol 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:
1. The blood-alcohol level was declining and the rate of elimination is known.
2. That the "burn-off" rate was .015 percent per hour.
How does the prosecution know that the defendant was eliminating (assuming he was eliminating) at that rate and not at .005 percent or .3 percent! Quite simply, the prosecution does not know: It merely assumes that the defendant eliminates at the average rate. And, of course, error in such an assumption translates into error in the extrapolation.
This ubiquitous "average person" in the DUI arena is not limited to chemical analysis. We even find him with the arresting officer in the field. When the officer administers the increasingly common "horizontal gaze nystagmus'' test as part of the battery of field sobriety tests, he operates on the assumption that the suspect is "Mr. Average." The officer has been trained to "read" at what angle the suspect's eyes begin jerking. A blood-alcohol reading can theoretically be obtained by subtracting the angle from 50; jerking at 35 degrees, for example, would mean the suspect has a blood-alcohol level of .15 percent. Where does the magic figure of 50 come from? The average person.
An alternative method of administering the nystagmus test is to "flunk" the person if jerking begins before 40 or 45 degrees. Why? Again, because the average person would theoretically have .10 or .05 percent alcohol in his blood at this point.
In either test, of course, we do not know what the individual's actual "baseline" is--that is, the angle at which his eyes would begin jerking if he were sober. In both cases, the individual is assumed to be physiologically identical to the theoretical "average" person.