How accurate are field sobriety tests?

Currently, US law enforcement conduct Field Sobriety Tests (FST), a set of physical tests including Nystagmus detection (also called Horizontal Gaze Nystagmus, or HGN). FSTs are admissible in court, as a primary measure of impairment. NHTSA studies report that an accuracy of only 77% is observed on the field, principally due to improper training to successfully conduct the test.

The key study is Tharp, Burns, and Moskowitz (1981) Development and Field Test of Psychological Tests for DWI Arrest, Southern California Research Institute, National Highway Traffic Safety Administration & US Department of Transportation.