Field sobriety tests are commonly classified into two groups:
Standard Field Sobriety Tests
SFSTs consist of three evaluations developed by the National Highway Traffic Safety Administration (NHTSA). The three tests are:
- Horizontal gaze nystagmus (HGN)
- Walk and turn
- One-leg stand
HGN
Horizontal gaze nystagmus is an involuntary jerking of the eye that occurs naturally as the eyes move to the side. Frequently, this test is conducted by moving an object (pen or finger) side to side in front of a person’s face, and asking them to follow it with their eyes. The head must remain stationary. The examiner looks for three indicators of impairment:
- If the eye cannot follow the moving object smoothly
- If jerking is distinct when the eye is at maximized deviation
- If the angle at which jerking starts is within 45-degrees of center
Walk and Turn
The walk-and-turn test is conducted by having a person take nine steps, heel-to-toe, along a straight line, followed by a turn on one foot and nine steps back in the same direction. The examiner looks for eight indicators of impairment. If the subject:
- Cannot keep balance while listening to instructions
- Begins before instructions are finished
- Stops while walking to regain balance
- Does not touch heel-to-toe
- Steps off the line
- Uses arms to keep balance
- Makes an improper turn
- Takes an incorrect number of steps
One-leg Stand
The one-leg stand test is conducted by having a person stand with one foot approximately six inches off the ground and count aloud to a specific number, or until told to stop. The examiner looks for four indicators of impairment:
- Swaying while balancing
- Using arms to balance
- Hopping to maintain balance
- Putting the foot down.
Non-standardized Field Tests
NSFSTs are classified as any other mental or coordination test an officer may give to a subject in an effort to determine impairment. These commonly include:
- Rhomberg balance test
- Finger to nose test
- Finger count test
- Hand pat test
- ABCs
- Numbers backward test