Missouri police officers are trained to the National Highway Transportation Safety Administration standards. The NHTSA standardized field sobriety tests are a battery of three tests.
The first test in the battery is the Horizontal Gaze Nystagmus. Nystagmus is an involuntary jerking of the eye. Horizontal gaze nystagmus refers specifically to the jerking of the eye when the eye looks to the side. There are dozens of types of nystagmus and police officers are not usually adequately trained to tell the difference between them. In addition to alcohol, brain damage, depressants, stimulants, fatigue, lighting, vertigo, and other common medical conditions can cause nystagmus. Additionally, five to seven percent of the population has naturally-occurring nystagmus, that is without a known cause. The three clues officers look for are 1) lack of smooth pursuit; 2) distinct nystagmus at maximum deviation; and 3) onset prior to 45 degrees.
The second test in the battery is the walk-and-turn. The walk and turn is a psychophysical test used to test both the physical impairment and cognitive impairment of the subject. The subject must listen to the directions, placing their feet in the correct position, then walking nine heel-to-toe steps down an (often imaginary) line, before turning and walking nine steps back.
The third test is the one-leg-stand. This is also a psychophysical test used to test both the physical impairment and cognitive impairment of the subject. The officer is watching your balance, ability to follow directions, and the ability to gauge time.
The tests are rarely done properly. Officers make mistakes out of familiarity, expediency attempts, training issues, or simply a predetermined belief that the subject is drunk.