The following is reprinted from Appendix A to Development of a Standardized Field Sobriety Test (SFST) Training Management System, DOT HS 809 400, November 2001, a NHTSA Publication.
Standardized Field Sobriety Testing
The Standardized Field Sobriety Test (SFST) is a battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest. These tests were developed as a result of research sponsored by the National Highway Traffic Safety Administration (NHTSA) and conducted by the Southern California Research Institute. A formal program of training was developed and is available through NHTSA to help law enforcement officers become more skillful at detecting DWI suspects, describing the behavior of these suspects, and presenting effective testimony in court. Formal administration and accreditation of the program is provided through the International Association of Chiefs of Police (IACP). The three tests of the SFST are:
- Horizontal Gaze Nystagmus (HGN)
- Walk-and-Turn (WAT), and
- One-Leg Stand (OLS).
These tests are administered systematically and are evaluated according to measured responses of the suspect.
Horizontal Gaze Nystagmus is an involuntary jerking of the eye that occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his or her eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.08 or greater. NHTSA research found that this test allows proper classification of approximately 88 percent of suspects (Stuster and Burns, 1998). HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.
Walk and Turn
The Walk-and-Turn test and One-Leg Stand test are “divided attention” tests that are easily performed by most unimpaired people. They require a suspect to listen to and follow instructions while performing simple physical movements. Impaired persons have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.
In the Walk-and-Turn test, the subject is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The examiner looks for eight indicators of impairment: if the suspect cannot keep balance while listening to the instructions, begins before the instructions are finished, stops while walking to regain balance, does not touch heel-to-toe, steps off the line, uses arms to balance, makes an improper turn, or takes an incorrect number of steps. NHTSA research indicates that 79 percent of individuals who exhibit two or more indicators in the performance of the test will have a BAC of 0.08 or greater (Stuster and Burns, 1998).
One Leg Stand
In the One-Leg Stand test, the suspect is instructed to stand with one foot approximately six inches off the ground and count aloud by thousands (One thousand-one, one thousand-two, etc.) until told to put the foot down. The officer times the subject for 30 seconds. The officer looks for four indicators of impairment, including swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down. NHTSA research indicates that 83 percent of individuals who exhibit two or more such indicators in the performance of the test will have a BAC of 0.08 of greater (Stuster and Burns, 1998).
When the component tests of the SFST battery are combined, officers are accurate in 91 percent of cases, overall, and in 94 percent of cases if explanations for some of the false positives are accepted (Stuster and Burns, 1998).
The original NHTSA research found different accuracies for the SFST Battery than reported in the more recent study. Tharp, Burns, and Moskowitz (1981) reported accuracies of 77 percent for the HGN, 68 percent for the Walk and Turn, and 65 percent for the One Leg Stand components; 81 percent of officers’ arrest decisions at 0.10 BAC were correct when all three measures were combined. In contrast, Stuster and Burns (1998) found greater accuracies in making arrest decisions on the basis of SFST results in their study at 0.08 percent BAC, as described previously and summarized in the following table.
Comparison of SFST Accuracies 1981 vs. 1998
Study: Combined Tharp, Burns, & Moskowitz (1981)
Study: Stuster & Burns (1998)
The greater component and overall accuracies found during the 1998 study are attributable to 17 years of law enforcement experience with the SFSTs since the original study and a lower criterion BAC than in the original study (i.e., 0.08 vs. 0.10 percent).
Horizontal Gaze Nystagmus (HGN)
A Resource Guide for Judges, Prosecutors and Law Enforcement