Despite it common use at DUI stops, there are a number of external factors that can impact field sobriety testing. These factors include the following:
Age: Yes, it's true; a person's age does have an impact on how a person performs on field exercises. This is recognized by the NHTSA (National Highway Traffic Safety Administration). According to their police training manual, people over the age of 65 should not be given the tests.
Foot wear: Makes sense don't you think? Certainly it's much harder to perform a one leg stand or a walk and turn exercise in high heels than in Dockers. Have you lawyer bring your shoes to court. You'd be surprised how effective this one little thing can be.
Disease: There are a multitude of diseases that can impact one's ability to perform field tasks, including inner ear disease or prior head trauma. If you have a really bad cold, and your ears are plugged up, this can impact your balance because that's where balance happens.
AD/HD: If you have attention deficit disorder, and the officer knows it, and gives you the test anyway, you can use this fact to possibly win the case. A person with AD/HD can't be expected to listen to complex instructions and perform well at the road side.
Nervousness: People respond to the presence of the officer in different ways, and being nervous can impact your physical and mental ability. If this were not true than why would Olympic athletes sometimes choke, and blow the medal when they've done the same thing millions of times before?
Flashing Lights: The officer may have given you the HGN test (Horizontal Gaze Nystagmus). If this was given while you were facing the flashing lights, such lights can actually cause the nystagmus (involuntary jerking in the eyes) to occur.
Improper Constitutions: The NHTSA rules require that you be given the tests under reasonable circumstances, meaning for example, a reasonably dry flat surface. Bad conditions can lead to bad results.
Sleep Deprivation: Studies have shown that people who are sleep deprived look just like people who are intoxicated. Sleep deprivation can cause balance and memory problems, and if it's severe enough, can even cause slurred speech.
Test Improperly Administered: Just as NHTSA requires certain conditions, NHTSA also requires that the tests be administered according to the standardized protocol. If they are not, the results are invalid. You can't be expected to perform properly on tests with bad or confusing instructions.