The National Highway Traffic and Safety Administration has endorsed three tests for law enforcement officers to use in the event that they suspect individuals of drunk driving. Those three tests are the horizontal gaze nystagmus, walk and turn and the one-leg stand. Police officers in Wisconsin use these tests as well as others to judge whether drivers are intoxicated; though NHTSA research suggests that failure to perform on the three mentioned tests does generally indicate intoxication on the part of a driver, no test is infallible and each may subjectively affect or challenge some individuals more than others.
For example, there are a host of medical conditions that may impact a driver's ability to perform one or all of the mentioned tests. A driver with balance or coordination problems may not be able to successfully complete the walk and turn test or the one-leg stand. A driver with an eye condition may not be able to maintain eye control to such an extent to pass the horizontal gaze nystagmus.
Though these tests may be subject to standardization protocols promulgated by state and federal agencies, how individuals with subjectively different physical characteristics perform on them can be very different. Since a police officer may not be able to tell that a driver suspected of drunk driving suffers from a medical condition that may impact the person's performance on field sobriety tests, the officer may arrest an individual who is not actually breaking any drunk driving or DUI laws.
Subjectivity in terms of a suspect's physical and mental conditions may greatly influence how a person performs on a field sobriety test. An arrest for drunk driving may result even if a person is fully sober at the time of the field sobriety testing; to overcome the apparent evidence of intoxication that a field sobriety tests indicate, people may choose to defend themselves in court to overcome the inaccuracies regarding their conditions noted during their arrests.