In 
this post, I tried to provide a basic understanding of a DUI incident.   In Part II, I will focus specifically on the voluntary field sobriety  evaluations.
Please note that I referred to them as both as voluntary and as  evaluations.  By voluntary I mean just that.  A driver has no obligation  to perform the evaluations.  Failure to do so is not an indicator of  intoxication.  In fact, some people will not be able to do them due to  medical screening issues built into the evaluations.  As for the term  evaluations, they are commonly termed as “tests”, but this is not  accurate as “tests” indicates that they are on a pass/fail basis, which  is not correct.  They are an evaluation tool that allows the peace  officer to make a determination based on the totality of the  circumstances.
The actual correct term for the evaluations is Standardized Field  Sobriety Evaluations (SFSE) with emphasis on the “standardized”.  The  evaluations, at least the standardized version of them, are  scientifically proven and accepted.  They should be completed in the  prescribed order.  There are only three SFSEs.  They are the Horizontal  Gaze Nystagmus, Walk and Turn, and One Leg Stand.  Any evaluation used  other than those three are non-standard.  A peace officer must  successfully complete a training course to be certified to conduct the  standardized evaluations.
Nystagmus is an involuntary jittery or twitching eye movement.  While  some nystagmus may be natural, it is readily induced by alcoholic  beverages and some drugs.  As it is not a voluntary movement, it is not  able to be controlled by the individual; so, it is a very telling  evaluation.  It does require several medical screenings for the  evaluation to be valid.  For instance, a person who does not have equal  pupil size or their eyes do not track equally is medically screened out  from participating in this evaluation.  Nystagmus is checked for by  passing a stimulus back and forth at a distance of 12 to 18 inches from  the eyes.  The first several passes are to conduct the medical  screenings.  The rest are to check for smooth pursuit, nystagmus at  maximum deviation, and the onset of nystagmus at prior to a 45 degree  angle of the eyes.  As for smooth pursuit, the eyes should smoothly  follow the stimulus as it moves back and forth and not exhibit jerking  movements, which are an indicator of intoxication.  As for maximum  deviation, an intoxicated person is likely to show sustained and  distinct nystagmus as the stimulus is held at the furthest point of the  back and forth passing of the stimulus.  Finally, the onset of nystagmus  prior to the eyes reaching a 45 degree angle when following the  stimulus.  Vertical nystagmus is also checked, but vertical nystagmus is  not an indicator of drugs as is commonly believed.  It is an indicator  of a high intoxicant level for the particular person.  Training for what  to look for in the eyes is conducted through the use of videos of the  eyes of people at various intoxication levels.  The use of live drinkers  is now discouraged for this training.
The Walk and Turn evaluation is an evaluation of both a person’s  ability to follow directions as well as their ability to walk in a  straight line and maintain their balance.  Obviously, there are medical  conditions that could prevent a person from being able to complete this  evaluation; so, it is important for the officer to ask good screening  questions.  The exercise should be demonstrated by the officer prior to  the driver participating.  Of note here, I have bad ankles due to a  chronic medical condition as well as an injured knee.  Prior to  demonstrating this evaluation, I always informed/inform the driver of  this so that they had a better perspective of the evaluation and the  physical ability needed to do complete it.  This evaluation involves the  participant beginning from a prescribed position, taking nine steps  touching heel to toe with each step, turning in a prescribed and  demonstrated manner, and then taking nine heel to toe steps back to the  starting point.  The line walked should be straight with the  participant’s arms by their side and with their eyes watching their feet  as they walk.  The officer is looking to see if the driver steps off  line, fails to touch heel to toe with each step, and whether or not the  driver raises their arms from their side to maintain their balance.
The One Leg Stand evaluation involves the driver raising one foot  approximately six inches off of the ground and with the toe pointed.   Their arms should be at their side, their support leg should be locked  at the knee, and their eyes should be focused on the toe of their  extended foot.  Like the Walk and Turn, there are medical/physical  conditions that could legitimately prevent a person from being able to  participate in the evaluation.  In this evaluation, the officer watches  to see if the driver is able to maintain their balance without hopping  or swaying and without touching their foot back to the ground.
The above are the standardized evaluations and they should be  completed in the order as described.  There is no pass/fail point at  which a person is declared intoxicated to the point of being DUI or not.   The officer makes a determination based upon everything observed  during the contact to include slurred speech, the odor of an alcoholic  beverage on the driver’s breath, and their coherence, appearance and  demeanor.  Again, they are completely voluntary.  If a driver chooses  not to participate, the peace officer must make a determination at that  point whether or not to proceed with an arrest.  I do not have  statistics to reflect any specific percentage as to how many times I  made an arrest after conducting the exercises versus letting a person  go.  The latter category would be the higher percentage.
There are many non-standard evaluations that can also be used.  There  is a myth that just will not be put to rest when it comes to one of  these.  Whether or not a person can say the alphabet backwards is not an  indicator of intoxication.  I do not know how many times when asking a  person if they would participate in voluntary SFSEs tell me that they  would do so but they wanted to let me know up front that they could not  say the alphabet backwards.  I usually try to keep a straight face and  say something along the lines of, “Okay, we’ll skip that one then.”  I  did/do commonly ask people to say (not sing) the alphabet.  I never had a  person mess up the alphabet that did not provide a considerably high  blood alcohol content sample.  Of course, in order for this evaluation  to have any merit, it must be established that the person in question  knows the alphabet.  I did have one occasion when a Columbian national  stated that he did not know the alphabet in English.  I hope that Mrs.  Bailey would be proud to know that many years after high school Spanish  class that I still remembered the Spanish alphabet and was able to  complete the evaluation.
Click 
here  to see a form that we use when conducting SFSEs to help ensure that the  evaluations are done correctly.  It accompanies a DUI arrest report.
I hope that the above helped provide you with a clearer picture of  field sobriety evaluations.  It is a difficult topic to tackle without  getting into the minutia of the details, and some aspects of the topic  just do not easily translate into text.