BLOOD ALCOHOL TESTS
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.
Breath-Alcohol Testing Definition
An encyclopedia definition of breath tests that are given to detect alcohol content in the body.
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