The National Highway Traffic and Safety Administration (NHTSA) has done extensive studies of field sobriety testing (FST's) in order to determine the probability of intoxication. Law enforcement agencies have utilized FST's for many years to develop probable cause for arrest and as evidence to be used in the prosecution for drunk driving. However, not all FST's have been scientifically determined to be accurate reflectors of alcohol intoxication. NHTSA has determined that three FST's are accurate indicators of alcohol intoxication, when administered in a standardized manner. These tests are called Standardized Field Sobriety Tests (SFST's):

1. Horizontal Gaze Nystagmus (HGN). HGN is an eye test that is 77% accurate in reflecting a blood alcohol count (BAC) above .10%. Nystagmus is an involuntary jerking of the eyes at a certain angle, off to the side.
2. Walk-and-Turn (WAT). WAT is a walking test that is 68% accurate in reflecting a BAC above .10%.
3. One-Leg Stand (OLS). OLS is a test done in a stationary position that is 65% accurate in reflecting a BAC above .10%.

The HGN in combination with the WAT reflects an 80% accuracy rate in determining a BAC above .10%.

The WAT and the OLS are "divided attention" tests. Divided attention tests require a subject to listen to and follow instructions while performing simple physical movements. Sober individuals are supposed to perform these tests with no problem. Impaired individuals, however, have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.

As mentioned above, law enforcement agencies utilize FST's in addition to SFST's. The SFST's have an advantage in that they have scientific backing. However, FST's do not have such scientific data to support the conclusion that the drunk driving subject is intoxicated. Examples of FST's include: the Rhomberg Balancing Test, Finger to Nose, Alphabet Test, Count Down, and Hand Pat, among many others.

Even though the FST's do not have scientific backing reflecting reliability, most courts will still admit the tests into evidence. In California, courts allow all FST's into evidence, but defense counsel can attack the weight of the evidence by challenging the reliability of the non-standardized tests. Other states only allow the actual SFST's into evidence.

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