A police officer who stops you for the suspicion of drunk driving will have to try to ascertain if you are actually impaired. This can be rather complicated because some of the testing that must be done is subjective. Field sobriety tests can fall under this subjective label because the results are based solely on the interpretation of the results as noted by the officer.
Field sobriety tests are usually done before other tests, like Breathalyzers, are done. The standardized field sobriety test is a three-part test that includes a walk-and-turn, a horizontal gaze nystagmus and one-leg stand test. These tests are sometimes used in conjunction with non-standardized tests that could include having to recite the alphabet or touching your nose with a finger while your eyes are closed.