Field sobriety tests (FSTs) are performed by the police to establish probable cause to arrest a DUI suspect. Field sobriety tests are standardized tests. Therefore, it is crucial that they are administered correctly. If administered correctly, poor performance on the FSTs supposedly corresponds to a BAC of .10 or above.
Completion of FSTs is optional, and it may be to your advantage to decline to perform them. Read below to learn about the 3 most common FSTs, and how to defend against them.
HORIZONTAL GAZE NYSTAGMUS (HGN)
The HGN test is one of three “Scientifically Validated” field sobriety tests. During the HGN test, the officer tests whether or not the individual is able to smoothly track a stimulus with their eyes. The officer uses either their finger or a pen to see if the eyes are able to follow without involuntarily jerking. Like all other field sobriety tests, completion of the HGN test is optional.
Horizontal Gaze Nystagmus Administration
According to the National High Traffic Safety Administration (NHTSA) training manual, an officer must take the following steps when administering the HGN test:
Horiztonal Gaze Nystagmus Clues
When performing the HGN test, the officer looks for a total of six clues (three clues in each eye). According to NHTSA, if four out of six clues are present, it is likely the individual’s BAC is greater than .10. The six clues are:
Horizontal Gaze Nystagmus Flaws and Defenses
The HGN field sobriety test has two main flaws. First, it is often administered incorrectly. As you can see, there are many steps in administering this tests which are easy for an officer to overlook or rush through. The second main flaw of the HGN test is that there is a whole laundry-list of alternative explanations for the presence of nystagmus. These include, but are not limited to, lack of sleep and caffeine.
ONE LEG STAND (OLS)
Like all other FSTs, you are not required to perform the One Leg Stand (OLS). There are two OLS administration stages: instruction and balancing/counting.
One Leg Stand Administration
Instructions Stage and Verbal Instructions
The officer begins the OLS by giving the following instructions:
The instructions stage divides the subject’s attention between a balancing task (maintaining a stance) and an information processing task (listening to and remembering instructions). 
Demonstrations and Instructions for the Balancing and Counting Stage
The officer must further explain the test requirements as follows:
Observe the suspect from a safe distance. If the suspect puts the foot down, give instructions to pick the foot up again and continue counting from the point at which the foot touched the ground. If the suspect counts very slowly, terminate the test after 30 seconds.
This stage divides the subject’s attention between balancing (standing on one foot) and small muscle control (counting out loud). 
One Leg Stand Clues
If a DUI suspect shows two or more of the above clues, or fails to complete the test, the officer assigns a BAC of .10 or more. 
One Leg Stand Flaws and Defenses
The main problem with the OLS is that it’s difficult to perform sober. The OLS requires a certain level of balance that many people don’t have due to past injuries or poor coordination. On top of that, performing the OLS on an uneven surface can negatively affect one’s performance on this FST. Even if it’s performed in perfect conditions by an individual who is physically capable, the OLS is only accurate 65% of the time at classify a BAC of .10 or above.
WALK AND TURN (W+T)
The W+T test basically requires the suspect to take 9 steps in a straight line, then turn around and take another 9 steps in a straight line in the opposite direction. It is divided into two stages: instruction and walking.
The officer begins by having the DUI suspect get into a heel-to-toe stance by giving specific instructions and demonstrating those instructions.
NHTSA lists the following verbal instructions for the W+T test:
Walk and Turn Clues
If the officer observes two or more clues, there is a 68% chance the suspect has a BAC above .10.
Walk and Turn Flaws and Defenses
The analysis is similar to the above analysis of the OLS. Test conditions can negatively affect one’s performance on this FST. Also, officers are notorious for poorly instructing and demonstrating this FST to suspects.