Police will conduct standardized field sobriety tests or SFSTs on DUI suspects. There are three SFSTs called Horizontal Gaze Nystagmus (HGN), Walk and Turn (WNT), and One Leg Stand (OLS).
HGN is a test for involuntary movement of eyes as they gaze to the side. It requires the officer to ask if you have any eye problems. Then they will tell you to look at a pen or finger in front of your face. They look to see if your pupils are an equal size. Unequal pupils are evidence of eye or head injury and invalidate the test. Then they will move the finger or pen around to see if your eyes can track the stimulus equally. This is to rule out further eye problems that will interfere with the test. The officer should also face you away from traffic, moving objects, and flashing lights.
The HGN test looks for 3 clues in each eye for a total of 6 clues. The clues are for nystagmus while your eyes move horizontally, nystagmus while looking all the way to the side, and nystagmus before 45 degrees as your eyes look to the side. NHTSA publishes that 4 out of 6 possible clues can indicate that someone will test over the limit.
The WNT test has 8 clues. NHTSA says that any 2 of the 8 clues is an indication that someone will test over the limit. The test is basically walking heel to toe for 9 steps, turning as directed, and 9 heel-toe steps back.
The OLS test has 4 clues. NHTSA says that any 2 or more clues can indicate that someone will test over the limit. The test is performed with one leg raised off the ground 6 inches. The leg is held with the leg straight for 30 seconds.
These tests are highly regulated by NHTSA. Failure of the officer to give proper instructions or the making of inaccurate observations can cause the tests to be excluded.
Police will often use non-standardized tests. These tests are subject to suppression. This means that the law generally will prevent them from being used by the prosecutor against you at trial. However, it should be noted that the officer will be able to talk about what was observed even if he cannot use the words “test” or what it means to “fail” the test. Most common of these types of tests are counting, alphabet, finger to nose, and touching thumb to fingers while counting.
Counting tests are can be up or down. They are usually done as a supplement when a suspect has some problem performing the SFSTs. NHTSA does spell out some regulations about how to perform the test. They recommend that the number start with an unusual number like 68 and stop with counting down to another unusual number like 53. They look to see if you can count backwards, miss numbers, add numbers, and start and stop at the right place.
Alphabet tests are usually the same as counting. They are used as a supplement when someone cannot do the regular field tests. Some officer use the alphabet and ask that you not sing it. NHTSA instructs that the tests start at a weird letter and stop at a weird letter such as starting at E and going to P. Officers want to see if the person will skip or add letters while simultaneously checking that they remember to start and stop at the right place.