Field Sobriety Tests are psychophysical tests used by law enforcement to determine if a person is intoxicated and ought to be arrested for testing.
Psychophysical tests are methods of assessing a person’s mental and physical impairment. These tests focus on the abilities needed for safe operation of a motor vehicle: coordination, information processing, and so on.
The National Highway Traffic Safety Administration (NHTSA) has validated three standardized field sobriety tests. The three tests are: the Horizontal Gaze Nystagmus (HGN) test, the Walk and Turn test, and the One Leg Stand test. These tests are standardized because they employ specific instructions and an objective grading criteria. Other field sobriety tests include the Rhomberg balance test, the finger to nose test and the reciting of the alphabet.
Horizontal Gaze Nystagmus refers to an involuntary jerking occurring as the eyes gaze toward the side. In addition to being involuntary the person experiencing the nystagmus is unaware that the jerking is happening. Involuntary jerking of the eyes becomes readily noticeable when a person is impaired. As a person’s blood alcohol concentration increases, the eyes will begin to jerk sooner as they move to the side. When the Horizontal Gaze Nystagmus test is administered, the officer has the suspect follow the motion of a small stimulus with their eyes only.
The NHTSA research has found that this test is 77% accurate and declared the Horizontal Gaze Nystagmus test as the most reliable field sobriety test.
This means that the police are wrong 23% of the time.
The Walk and Turn test is administered by the officer instructing the subject to: take nine heel-to-toe steps, turn in a prescribed manner, take nine heel-to-toe steps back, while counting the steps out loud and watching their feet. During the turn the suspect is to keep their front foot on the line, turn in the prescribed manner, and use the other foot to take several small steps to complete the turn.
The Walk and Turn test is interpreted in a standardized manner. Officers administering the Walk and Turn test observe the suspect’s performance for eight clues:
- Can’t balance during instructions
- Starts too soon
- Stops while walking
- Doesn’t touch heel to toe
- Steps off line
- Use arms to balance
- Loses balance on turn or turns incorrectly, and
- Takes the wrong number of steps
Inability to complete the Walk and Turn test occurs when the suspect:
- Steps off the line three or more times
- Is in danger of falling
- Cannot do the test
Original research shows that if a suspect exhibits two or more of the clues or cannot complete the test, the suspect’s Blood Alcohol Content (B.A.C.) is likely above .10. This criterion has been shown to be accurate 68% of the time.
This means that the police are wrong 32% of the time.
The One Leg Stand test consists of two stages, the instruction stage and the balancing & counting stage. During the instruction stage the subject must stand with their feet together, keep arms at their sides and listen to instructions. In the balance and counting stage, the subject must raise one leg (left or right) with the foot approximately six inches off the ground, keeping the raised foot parallel to the ground, and while looking at the elevated foot, count out loud in the following manner: “one thousand and one”, “one thousand and two”, “one thousand and three” until told to stop. The officer usually times the test for 30 seconds. The One Leg Stand test is also administered and interpreted in a standardized manner.
Officers carefully observe a suspect’s performance and look for four specific clues:
- Sways while balancing
- Use arms to balance
- Hops
- Puts foot down
NHTSA’s research shows that when the suspect produces two or more clues or is unable to complete the test, the suspect’s Blood Alcohol Content (B.A.C.) is likely above .10. This criterion has been shown to be accurate 65% of the time.
This means that the police are wrong 35% of the time.
There are certain physical factors that further compromise the validity of these field sobriety tests. Certain medical conditions and physical injuries can invalidate the results. If a person is 50 pounds overweight or is 65 years or older, then the results of these psychophysical tests are invalid.
