Critique of blood alcohol readings
One of the greatest sources of error in blood alcohol testing is the misconception that an individual being tested is perfectly average in certain physiological traits. Put another way, obtaining an accurate blood alcohol reading is dependent on the validity of a number of assumptions that are based on the average person.
An example of one of these assumptions is that the ratio between alcohol in a person's blood and alcohol in their breath (the "partition ratio") is 2100 to 1, which is the ration of the average person. Breathalyzers, devices which measure the amount of alcohol in a person's breath, are designed to produce a reading based on that ratio. However, the actual ratio in any given individual can vary from 1700:1 to 2400:1, or even more widely. Thus a person with a true blood alcohol level of .08 but a partition ration of 1700:1 would have a .10 reading on an "accurate" breathalyzer.
A similar assumption is required for urinalysis. When a subject's urine is analyzed for blood alcohol, a presumption exists that there are 1.3 parts of alcohol in the urine for every 1 part of alcohol in the blood. This 1.3:1 ratio is as fallacious as the 2100:1 ratio; it is also based on the average person, even though the actual ratio found in any given individual can vary greatly.
Yet another example is retrograde extrapolation. If an individual is arrested for driving under the influence and their blood alcohol level is tested at a later time, the reading at the time of testing will not be relevant to the charge. Therefore it is necessary to estimate what the blood alcohol level was at the time the individual was driving. This is commonly done by extrapolating backward - that is, computing the earlier blood alcohol level by estimating how much alcohol had been eliminated or "burned off" in the interim between driving and testing. This relies on two assumptions: that the blood alcohol level was declining, and that the rate of elimination is known; the rate for the average person is .015 percent per hour. This rate doesn't necessarily apply to everyone, though, and an error in such an assumption translates to an error in the extrapolation.
In addition to the assumptions which affect chemical tests of blood alcohol content, there exist similar assumptions in field testing. When a police officer administers the "horizontal gaze nystagmus" test as part of the battery of field sobriety tests, he attempts to read at what angle a 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. Just as with the chemical tests, this is based on the angle for the average person, which doesn't necessarily apply to everone.
Don Nichols, one of the foremost DUI lawyers in the country today, points out to juries, in appropriate cases, that his client is female, Chinese, and deceased--despite obvious evidence to the contrary. He then explains that statistically there are more women than men in the world, more Chinese than any other nationality, and more dead human beings than living ones. Statistically, then, the average person is female, Chinese, and deceased--and so, according to
the prosecution, must be his client. He usually also asks the jury how many of them have 2.3 children--the average in the United States.
Critics of blood alcohol testing argue that there are multiple erroneous assumptions being made, and that these assumptions are critical to the validity of a blood alcohol reading. As the computer technicians are fond of saying, "Garbage in--garbage out."
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