The high reported accuracy of the standardized field sobriety test is a property of the statistic not of the test

Abstract

Abstract OBJECTIVE In the 1990s as the legal blood alcohol limit for driving changed, validation studies reported the Standardized Field Sobriety Test (SFST) to be accurate at discriminating between Blood Alcohol Concentrations (BAC) above or below several legal limits: 0.10, 0.08, 0.05 and 0.04%. We investigated the contribution of the validation studies’ choice of accuracy statistic to the SFST’s reported accuracy. METHODS Using the data set from a commonly cited SFST validation study, we calculated the arrest accuracy and overall accuracy of the SFST at identifying BACs above or below 31 different target BACs from 0.00 to 0.30%. RESULTS At target BAC 0.30% the arrest accuracy of the SFST is 1%; at BAC 0.15%, 34%; at BAC 0.00%, 100%. The statistics arrest accuracy and overall accuracy describe the SFST, a test designed to identify changes caused by alcohol, as less accurate when the changes are severe, more accurate when changes are mild, and as 100% (arrest) and 93% (overall) accurate when there are no changes at all. CONCLUSION The statistics overall accuracy and arrest accuracy to not quantify the probability that impaired driving defendants who failed the SFST had an elevated BAC or were impaired.