What is a field sobriety test?  In the 1970s, the National Highway Traffic Safety Administration developed some physical exercises that they believed were reliable indicators of alcohol intoxication.  In Iowa, officers primarily use three of these tests: horizontal gaze nystagmus (HGN), walk and turn, and one-leg stand.  Each of these tests is alleged to be a reliable indicator of intoxication, for example, the NHTSA claims the HGN by itself is 77% accurate, the Walk-and-Turn by itself is 68% accurate, and the One-Legged Stand by itself is 65% accurate.  These tests are problematic however.  One of the strict requirements of these “tests” is that the officer has no discretion in the manner in which the tests are to be performed.  In other words, in order for the test to have any meaning, it must be done exactly in the manner in which it was designed.  The authors of these tests specifically state in their training manual:

“IT IS NECESSARY TO EMPHASIZE THIS VALIDATION APPLIES ONLY WHEN: THE TESTS ARE PERFORMED IN THE PRESCRIBED, STANDARDIZED MANNER THE STANDARDIZED CLUES ARE USED TO ASSESS THE SUSPECTS PERFORMANCE THE STANDARDIZED CRITERIA ARE EMPLOYED TO INTERPRET THAT PERFORMANCE IF ANY ONE OF THE STANDARDIZED FIELD SOBRIETY TEST  ELEMENTS IS CHANGED, THE VALIDITY IS COMPROMISED.”

Officers very rarely perform the HGN in the manner required.  Another problem with the HGN is that there are dozens of things which cause nystagmus, not just alcohol.  And, moreover, there are dozens of types of nystagmus.  The Walk and Turn and One Leg Stand tests are difficult to be performed even by sober, athletic people in the best of circumstances on well lit, flat and dry surfaces free from debris.  These tests have been termed “divided attention tests”, and the officer is going to be looking not only at your physical behavior but how well you listened to the instructions to the test, assuming he gives proper instruction.  For example, at the beginning of the walk and turn he is going to say “don’t start until I tell you to start.”  He is then going to put one foot in front of the other and stand that way while he describes and demonstrates the test.  Most people are going to copy his behavior by placing one foot in front of the other at the same time he does it- his report will indicate that you are exhibiting signs of intoxication.  This is instinctual behavior, not realizing that you are being “graded” on this activity.  This one instance is just one example of the problems with these “tests.”  There has been additional study performed since the NHTSA developed these tests and they can be shown to be nothing more than impossible exercises designed for you to fail.