What Are Field Sobriety Tests?
Field Sobriety Tests (FST's) are psychophysical tests used to assess a person's physical and/or mental impairment. They focus on the abilities needed for safe driving. Most of the more reliable psychophysical tests are known as "divided attention" tasks. They require a person to concentrate on more then one task at the same time. To safely drive a car, a person needs to be able to simultaneously control steering, breaking, and acceleration; react to constantly changing driving environment; and perform many other tasks. Alcohol affects one's ability to adequately divide attention, thus causing drivers to concentrate on more difficult tasks while ignoring simpler ones (i.e. ignore a traffic signal while concentrating on one's speed). Even if impaired, most people can successfully concentrate on a single task fairly well, but when impaired, most drivers cannot successfully divide their attention between multiple tasks at once.
Divided attention tasks are designed to evaluate mental and physical capabilities a person needs to safely drive a car. They include information processing; short-term memory; judgment and decision making; balance; steady, sure reactions; clear vision; small muscle control; and coordination of limbs. A good FST will combine any two or more of these capabilities simultaneously. A test must also be reasonably simple for the average non-intoxicated person to perform.
The most common FST's used by the police include the three standardized tests consisting of the Horizontal Gaze Nystagmus tests, Walk & Turn test, and the One Leg Stand test. These three tests have been validated as reliable indicators of intoxication, although they are not 100% accurate. Other commonly used, but non-standardized, tests include counting backwards, saying the alphabet (or a portion of it), finger count, and the stationary balance (Rhomberg) tests.
In reference to the three standardized FST's, the government has admitted, and it is printed in the police officer's DUI manual, that "IT IS NECESSARY TO EMPHASIZE THIS VALIDATION ONLY APPLIES WHEN: THE TESTS ARE ADMINISTERED IN THE PRESCRIBED STANDARD MANNER; THE STANDARDIZED CLUES ARE USED TO ASSESS THE SUSPECTS PERFORMANCE; THE STANDARDIZED CRITERIA ARE EMPLOYED TO INTERPRET THAT PERFORMANCE. IF ANY ONE OF THE STANDARDIZED FIELD SOBRIETY TEST ELEMENTS IS CHANGED, THE VALIDITY IS COMPROMISED."