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.
In an interesting crime lab study reported in 25 Journal of the Forensic Society 476, 12 police officers measured the angle of onset of nystagmus in 129 actual cases where DUI suspects had been arrested but had not yet been tested by blood, breath or urine. The officers used a special protractor to help them accurately measure the anlge of onset. Result? Even with the aid of a protractor, they consistently over-estimated the angle at low BAC levels and underestimated it at high BACs. The researchers from the police crime lab concluded that nystagmus cannot be used to accurately predict blood alcohol concentration.