HGN and Tharp’s Equation

Tharps Equation is used in the DRE (drug recognition expert) protocol to determine whether or not alcohol is the cause of the observed nystagmus. In another words, it “rules” out alcohol as the cause of impairment. The formula the DRE candidate is taught is BAC = 50 – A (Angle of Onset).

The interesting thing about Tharps Equation is that it is merely a statistical approximation – not a mathematical formula (and we all know what Mark Twain says about statistics). Tharps Equation can be off by .05% or more, even if the person has consumed no drug other than alcohol. Even its proponents concede that Tharp’s Equation does not reflect an exact relationship for all subjects at all times (according to NHTSA).

Keep in mind that the SFST protocol only allows for the prediction of above or below the per se level. What usually happens is the DRE talks and unqualified officers use the formula to estimate the BAC (although 99% of the time they are not performing the Onset of Nystagmus Prior to 45 Degrees properly). We question the “reliability” of a statistical formula, as well as the proper estimation of 45 degrees by the Officer.
Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website

Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website

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