In trial, the prosecutor’s last question to a testifying officer is almost always "Officer, after these field sobriety tests, did form an opinion as to the defendant’s ability to safely drive his car?" The officer always answers, "I formed the opinion that the defendant was too impaired to safely drive a car."
The interesting thing about that opinion is the field sobriety tests were never validated to measure impairment. They were only validated to give the officer an estimate of whether the person is over the legal limit.
Marcelline Burns and Jack Stuster, in their study entitled "Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent" writes on page 27 of the study as follows:
"Many individuals, including some judges believe that the purpose of a field sobriety test is to measure driving impairment. For this reason, they tend to expect tests to possess "face validity," that is, tests that appear to be related to actual driving tasks. Tests of physical and cognitive abilities, such as balance, reaction time, and information processing, have face validity, to varying degrees, based on the involvement of these abilities to driving tasks; that is, the tests seem to be relevant "on the face of it." Horizontal gaze nystagmus lacks face validity because it does not appear to be linked to the requirements of driving a motor vehicle. The reasoning is correct, but it is based on the incorrect assumption that field sobriety tests are designed to measure driving impairment.
Driving a motor a motor vehicle is a very complex activity that involves a wide variety of tasks and operator capabilities. It is unlikely that complex human performance, such as the required to safely drive an automobile, can be measured at roadside.
It seems as though that the person that developed and validated these tests is saying the officers are incorrect when the officer says these tests measure impairment. The developers say these tests estimate a person’s blood alcohol content.