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a. HGN Testing
Horizontal gaze nystagmus is an involuntary jerking of the eyeball which 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 eyes.
The examiner looks for three indicators of impairment in each eye: 1) if the eye cannot follow a moving object smoothly, 2) if jerking is distinct when the eye is at maximum deviation, and 3) 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 0.10 or greater. NHTSA research indicates that this test allows proper classification of approximately 77 percent of suspects. HGN may also indicate consumption of seizure medications, phencyclidine and a variety of inhalants, barbiturates and other depressants.

b. Divided Attention Testing
The walk-and-turn test and one-leg stand test are “divided attention” tests that are easily performed by most sober people. They require a suspect to listen to and follow instructions while performing simple physical movements. This test is based on the fact that impaired persons have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.

c. Walk and Turn Test
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 seven indicators of impairment: 1) if the suspect cannot keep balance while listening to the instructions, 2) begins before the instructions are finished, 3) stops while walking to regain balance, 4) does not touch heel-to-toe, 5) uses arms to balance, 6) loses balance while turning, or 7) takes an incorrect number of steps. NHTSA research indicates that 68 percent of individuals who exhibit two or more indicators in the performance of the test will have a BAC of 0.10 or greater.

d. One-leg Stand Test
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 1) swaying while balancing, 2) using arms to balance, 3) hopping to maintain balance and 4) putting their foot down. NHTSA research indicates that 65 percent of individuals who exhibit two or more such indicators in the performance of the test will have a BAC of 0.10 or greater.
The effectiveness of SFST in court testimony and evidence depends upon the cumulative total of impairment indicators provided by the three-test battery. The greater the number of indicators, the more convincing the testimony. Because SFST is administered according to national standards and is supported by significant research, it has greater credibility than mere subjective testimony.

e. Alternative Testing Methods
Occasionally, certain suspects such as physically disabled persons are unable to perform the SFST and alternative testing methods must be employed. In such cases, some other battery of tests such as counting aloud, reciting the alphabet or finger dexterity tests may be administered. Several appellate court decisions have indicated that, if you administer a test that requires the subject to respond orally in other than a routine information-giving fashion, such as requiring them to indicate the date of their sixth birthday, and if they are in custody at the time, you should administer the Miranda warning first, because the officer is seeking information that is testimonial in nature.