Which of the following is not a validated field sobriety test?

The Modified Romberg Balance test is not a validated field sobriety test according to the guidelines established for standardized field sobriety testing. This test focuses on a subject's ability to maintain balance while standing still, often requiring them to close their eyes or lean their head back, which can be influenced by a range of factors unrelated to intoxication.

In contrast, the Horizontal Gaze Nystagmus, Walk and Turn, and One Leg Stand are specifically validated and widely accepted tests used to assess impairment due to alcohol or drugs. These tests were developed and researched to measure specific physiological responses that indicate impairment, making them reliable indicators in the context of DUI investigations.

The validation of the other tests lies in their ability to demonstrate consistent results correlating with blood alcohol concentration levels, establishing their relevance in the field sobriety testing protocol.