Nystagmus:
"The Eye Test"
(Part 3)
Information
courtesy of Lawrence Taylor - DUIblog
As
indicated in previous posts, there are three parts of the nystagmus
test. But it is the angle of onset segment which is most critical
-- primarily because the "distinct" nystagmus and "smooth
pursuit" in the other two tests are fairly subjective, while
an angle has a certain mathematical nicety to it.
However,
the officer's ability to estimate this angle is critical. The nystagmus
test is premised upon a formula that requires the angle of onset
to be subtracted from 50 to obtain a very rough estimation of blood-alcohol
concentration (BAC). An angle of 45 degrees from center, for example,
may indicate a possible .05% blood-alcohol concentration; anything
before that -- for example, 43 degrees indicating .07% -- results
in a "failure". Clearly, if the officer is mistaken in
his "guesstimate" by only 5 degrees, a true 47 degree
(.03% BAC) "pass" becomes an observed 42 degree (.08%
BAC) "fail".
So
how does the officer measure the angle of onset with precision?
He
doesn't. At best, he is giving a very rough estimate. Recognizing
the importance of the officer's skill in estimating angles of onset,
the National Highway Traffic Safety Administration recommends that
officers use an angle-measuring template and practice with four
or five subjects: "Check yourself monthly with the device to
be sure that your accuracy has been sustained."
The
simple fact is that no officer actually does this; the last time
he used a protractor to estimate angles was in the police academy.
The most common method actually used is to assume that the 45-degree
angle from the eye intersects the held object at the suspect's shoulder:
if nystagmus is observed before the pencil or finger reaches a line
projecting straight out from the edge of the shoulder, the suspect
"fails". Very simple. Of course, the fatal flaw to this
method (other than now requiring two estimations) is that we all
have different shoulder widths.
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